About tutorialization:
Hmmm I see what you're saying, and yes you are right, but there is literally no tutorial or in-game explanation for numerous things, countless, and the new tutorial is even shorter than the old (and outdated) one.
I'm talking about tutorialization in-game, not YouTube. I also learnt the game reading the patch notes, discussing in Discord, YouTube videos, practicing in sandbox/NYC... everything the same as you, but the in-game tutorialization has fallen back a lot over the years.
Tutorialization and matchmaking/rankings have fallen to low priority over releasing new heroes, reworks, balancing... things that I love and made the game great, don't get me wrong, but I believe it's about time for a pause on new heroes and new items and reworks and monetization/cosmetics and put tutorialization and matchmaking/rankings rework on top priorities for the next updates.
About the matchmaking and ranking experience:
I see that you have had a totally different experience with rankings and matchmaking and learning the game, I'd also have a different experience with it if I started playing last year or next year... and that's also part of the problem. Most players don't put 500 hours in 1 month like you nor do they want to learn the game on YouTube or Reddit... and neither my case, most players don't put two years testing the game.
Both times a big influx of new players came into the game (NDA lift and Old Gods New Blood) the quality of the actual games degraded noticeably. I guess it's somewhat holistic, it's not only a matchmaking problem, but how everything else sums to the problem we all experience and (wrongly) conclude it's just the MM being bad.
I kinda imagine devs were not expecting the amount of new players coming in when the NDA was lifted and with OGNB update... or maybe it really is better to leave those for last, whenever all heroes, items, and map reworks are actually settled?
What do I know, I'm just a peasant.
I'm speaking straight from my ass so don't take this seriously
For example, you might not have noticed but with every big wave of new players not only the skill disparity is noticeable, also the toxicity in both teams, things that they bring from their favourite games.
I can spot a newcomer League player, same with Dota or CS... just by the words they like to use and their behaviour in-game or even in this forum/discord, I'm generalizing like crazy and I know it's not 100% but it checks out most of the time, believe me I've asked 
League players are toxic by default, and they will blame the team always while freezing the lane with the enemy guardian still full HP at minute 20. Their favourite word is "cancer" and "kys".
CS players are toxic by default, and they just insult everybody because they still think "that's part of competitive gaming" just because they are sunk in a bad environment (trashtalk, banting, gambling and cheating provided by Valve's second stigmata, the third one is Arctifact and the first one is TF2) Their favourite word is screaming at the mic whatever heinous stuff they heard that day in premier, but I appreciate that they normally insult the enemy team more than their own. Their favourite heroes are obviously Haze and Vindicta.
And Dota players are not toxic by default but they will try to bend the game to what they already understand from Dota, and bash their head against the same wall not understanding that in some ways this is a different game and everything is subject to change and maybe some of those preconceived ideas from Dota don't turn out to be so fun in Deadlock. Favourite word is a sudden "blyat" in-between an English phrase. They are kinda wholesome if you ask me.
I'm obviously kidding.
I find it funny, (yes im going bottom up) that you have that description for league players where
As i find myself (coming from league peaked GM) that my teammates are normally the one's who are AFK or not present in lane, therefore creating this 2v1 environment, and either my skill level far surpasses that of my laner and he then decides to not play simply bc he doesn't know how to play with what he's witnessing OR he defaults to thinking that i am an extremely bad player and i am only trying to get kills and I'm ignoring economy. Either/or The playerbase makes it very hard for a Winner's mentality to really shine especially with how close minded the community and honestly the world is as a whole nowadays.
The influx of new players will always deter the "Correct" statistics from being shown, but this is normally accounted for as it's not only a statistic for how the game is going, but also (theory crafting) The health of the game. What i mean by this, is how does a new surge of players affect the current players and does it create more discrepancies or does it create or more consistent influx of players.
[I'm a bum idk how to make my text all fancy, but further adding on to Matchmaking]
A lot of people overlook the actually Rating system in this game and don't truly understand it as well. And i should have mentioned earlier. But THESE things matter. Each and every Hero has a skill Rating, so While you do have an overall Rank, the lobbies are specifically designed for the appropriate skill Level that the game determines you on whatever Character it is you are playing.
Something No other game I've seen does, the closest reference i could think of is Overwatch Splitting up Tank, Damage, and Support into different skill brackets. Deadlock has just made it much more simplified and not so stressful on the eyes.
(Also i learned Damn near everything thru strictly gameplay, and I only ventured into youtube when i started to get curious if there were any other ways to accumulate souls/more information on items.
I do believe, that if you are more successful and many characters, then your rank will Rise faster than if you simply are Really good on one character and then not very good on others. Just food for thought tho and i have no actual idea as I prefer to keep my mind locked away in New York xd
AS FAR AS TUTORIAL TALK
Why i understand what you're saying, good points btw, i just. I don't know, coming from a playing a moba for 10+ years, I just can't agree with you. As far as matchmaking/rankings, whatever. But as far as them Adding more onto the tutorial. I get it, it's a hard game, moba's normally are. Youtube has kinda ruined it, amongst many other sites that exist nowadays, but honestly at it's core. Learning how the Macro aspects of the game work is kinda part of a player's skillset as well as just part of the fun.
I guess everyone's different, but personally if they taught me everything in the tutorial I would be extremely bored with the game, because they would essentially kill my ambition for learning and that would ultimately just take away a part of the game that makes it fun.
Personally I think the game is absolutely fucking perfect, Obviously it isn't but if i was to request any focus to shift. It would be geared towards trying to find ways to neutralize some of the net jitter that persists in these lobbies. I can enjoy everything, even dying 20 times. But i can't enjoy not being able to play. THAT is the WORST feeling. And I know that is a TOUGH Issue to fix for everyone. I'm just saying if i had to request a focus. It would be less of trying to improve, and more of trying to Refine.
P.S.
Also, as a league player, innately i get irritated easily, but that is a flaw of my own simply because i care too much of the actions of others and simply care for myself but not at all. Maybe that's a league player thing, but i think it's deeper than that. I'd like to consider myself a pretty amazing player to play with, if you can take me for what I am.
I love to banter the enemies in all chat, my team will see it as complaining. I see it as invigorating and I love turning around games that most people would consider losses. But people just, don't know how to enjoy games anymore. It's either Win or Tilt for most people and it's really the biggest issue with "matchmaking" If players simply cared less about scores and more so about others then I'd say everything could be just a tad bit better,
~but what do i know, I am simply, Sorrow.