Zandronum

The Zdoom Wiki covers everything, and by everything, it practically covers gzdoom's Decorate Doomscript/Zscript, and ACS to it's most recent version. As far as I know, the most recent version of Zandronum (as i post this) 3.0/3.0.1 supports GZDoom 1.8.6., which is years away from its most recent version 4. As such, I end up encountering incompatibility issues when i accidentally implement certain commands/scripts that postdate Zandronum's supported version.

Is there a way or any help to know everything that was accessible to 2.8pre-441-g458e1b1/GZDOOM 1.8.6?

You can go into the ZDoom Wiki, then use the "view history" button at the top right of a function page (like A_CheckProximity in DECORATE or floor in ACS for example), then look for a version of the page that's from May 2014 (or the closest that's not any newer than that), and in the vast majority of cases you'll see the feature exactly as Zandronum supports it, or if you can't find a page from May 2014 or older then it's not a supported Zandronum feature.

There are exceptions, though. I think A_CheckSight has a few flags that have been backported from newer GZDoom versions, and MODELDEF has support for SurfaceSkin (not even GZDoom 1.9.1 had this), but they aren't many.