Whereas GZDoom maintains a decent level of backwards compatibility with content made in the past, there are some issues that are bound to happen when running old ZDoom wads on GZDoom of today. The examples I had encountered are listed as follows:
-Phocas Island 2: at one point in the game, you can descend a staircase which leads to an icy world. However, due to some kind of bug, in modern GZDoom that staircase will teleport you into the deadly void if you attempt to descend it. Another problem is the fact that in the forest level, the beehives are stuck inside the ceiling, which means they are not functioning properly.
-Happy Time Circus 2: outright crashes in GZDoom, so I had to play it in Zandronum.
-Daedalus: Alien Defense: for the most part it worked fine, however, there were a few spots where a security camera actor was placed, and more often than not the camera would be stuck in the ground, or the ceiling, or be misaligned in general.
-Eternal Doom IV: might not be related to ZDoom in and of itself, but whereas most maps have a defined jump allowed or disallowed set up, there is no such thing for crouching. As a result, you can get softlocked if you crouch in the wrong area and enter an inescapable pit. As a matter of fact, the wad was released after crouching was added to ZDoom(circa 2005), though maybe the devs didn't know about that. However, there is an area in The Shrine where you can get teleported to a secret area, and leaving is impossible...unless you crouch. Whether this is a collision problem, related to the source port, or a level design problem, I do knot know!
-Action Doom: maybe I just had bad luck, or there is a way around this, but I couldn't defeat the first tank in the first level, because the turret head was unhittable, from what I remember.
I presume other people may have encountered similar problems, not limited to the above. The reason I am inquring into this is because whenever I'm going to play Mass Mouth, Ghoul's Forest or some other old ZDoom mod, it would be pretty bad if I encountered into unintended effects without realising it.
So, the solution is to probably use an older version of (G)ZDoom. How old, or how new, however, is the actual question. I would like to be able to play in 1440p, after all(Zandronum doesn't seem to have an option for that). If there are some compatibility measures modern GZDoom can use for cases like this, it would be good to know too.