There's a lot of different small incompatibilities between GM<=8.1 and GMS, mostly caused by the fact that pre-GMS versions didn't need to have the games run on any non-Windows platforms, with some of practices used (screen_redraw/screen_refresh/sleep/..) just not really being a thing on a fair of GMS' target platforms.
Also GM<=8.1 was quite careless about what would normally have been a decent cause of an error - for example, you could attempt to change a variable of a non-existent instance and this would raise no errors whatsoever.
Last year I've made a basic "at least this compiles"
update for someone, but I don't know how well it works nowadays (as there have been additional breaking changes since then).
For the health of modding in general it would be kind of cool to do a major rewrite of the game (for optimizations, bug fixes, and readability) but that is weeks and weeks of work and I don't think that anyone would consciously leap onto such adventure.