This issue has been known about and ignored for over a year now. I doubt they'll ever fix it at this point and it'll likely be up to modders to do so.
The issue is pretty easy to fix. The UI system Valve uses (Panorama) is based off of HTML/CSS, where element styles/sizes/padding/margins/etc are defined in the files. The problem stems from two things:
- The healthbar is positioned relative to the center of the screen, rather than the side.
- The UI system appears to scale things based on vertical size.
The healthbar's position is set up by placing it in the middle of the screen, then adding a stupidly high right padding value to "push" it to the left. You can see it in this picture here, where the blue stuff is the element itself and the yellow stuff is the padding:
The reason it disappears is because Panorama always assumes you're running a widescreen monitor and scales things as such. If, like me, you're running a 1600x1200 monitor, it just takes the vertical height of 1200 and assumes you're running a 1920x1200 or something like that. It tries to display the healthbar and all its padding onto screen space that doesn't exist, and I guess Panorama's fallback when this happens is to not display the element at all.
The easy fix is to position the element relative to the left of the screen. If you position it this way such that it matches its position on 16:9 monitors under the current system, the healthbar will still render on 4:3 displays, albiet on the character's back rather than next to them. I had made and was using a basic mod that did this, which allowed me to play on 4:3 just fine. Part of the reason I stopped playing up until this recent update was that I had to keep "remaking" this mod every time a patch dropped and something changed in the UI, and it was a huge pain in the arse for something shouldn't be a problem in the first place IMO.