The great regression begins at 1.1.0.0. If you're looking at the best compatibilty, prepare to have TWO Half-Life folders for before/after this version (maybe even three for the later steam ones)

Correction: it looks like I'm running 1.1..0.6 - Blue Shift/Opposing Force must have patched it up when I installed it. I'm using the "high definition" (well, 2001 high-definition) models pack that came with it, which is a bit of a novelty for me.

Is there a list somewhere of regressions/removed features with each version? I can only find one that focuses on features speedrunners use. I am getting the occasional line of dialog that sounds like it's clipping with EAX on (I have the volume set <50%), but nothing major.

So far I'm playing the Timeline trilogy "Remod" which was done in 2016 and updated for compatibility with newer Steam-based versions. It seems to run OK on 1.1.0.6 non-Steam. I haven't noticed anything 'wrong', but I'm completely unfamiliar with this pack and I'm not very far into it.

1.1.0.0 removed the old glquake model shadow code at least, but most importantly it had major network changes causing all the old mods to crash. Some mods were never updated to 1.1.0.0+ so watch out

The period of this happening was in Spring 2000 so most stuff after that point is safe.

Sorry, I can't advise you on what version of the original game (via patches) to use, but as Leileilol says, if you want to have the freedom to try all of the mods, then you'll need at least two different installations. I used to have two versions running, because of the mods I tried to run, but I can't remember the details because my HD drive crashed (and me, who advises everyone to back their stuff up, hadn't thought to backup the working and by then large HL folders...). I know one of the versions of HL was very old, the other was newer (with a different menu screen, but I can't remember what it looked like).

I don't actually know if all the mods worked properly with their (by me) allotted version of HL, as I only played all the way through a fraction of them, so if any of the rest loaded and ran for the minute or so that I tested them, then I assumed they'd probably work.

Anyway, I'd not played any HL mods in years, but some months back I was bored and so looked up current HL mods, and lots of people were recommending a new, amazing, maybe-the-best-ever HL mod, called Half-Life Echoes, so I tried it, and it really is as good as people say. It's fantastic, and you really should give it a go. For me it's up there with Azure Sheep and Opposing Force, and it's available from:

https://www.moddb.com/mods/half-life-echoes

I ran it using the fan-made game engine replacement for the official HL game engine, Xash3D, as the mod is very demanding and doesn't work on some versions of the official game. Xash3D might also run all mods (I dunno, but it probably doesn't, as life is never straight-forward, so it probably won't run some very early mods) but even if so then it might be too slow for a Windows 98 machine. But you should probably try it anyway, and you really should play Half-Life: Echoes, as it's fantastic, and it's all actually made by one person, surprisingly.

https://www.moddb.com/engines/xash3d-engine

Note that I cant remember which version of Xash3D I used to play through the Echoes mod, so this information might be redundant now, but when I ran Xash3D + Echoes, it was unstable, so after installing Echoes, go into the echoes folder, then open gameinfo.txt, edit the 16th line where it says max_edicts 900. Change 900 to 4096 or 2048 if that fails. I tried it with 4096 and I think that that value allowed me to play through the mod. Hopefull the later versions of Xash3D will have fixed this problem.