You are correct, the physics were tweaked in Foreverred to make the required tech for Trueskip more consistent. A lot of spikejumps in this version can randomly kill you if your position is misaligned. Fixing this let me get away with much more challenging level design in the remake!
That would explain why the normal Celeste Classic felt so weird to play after doing Foreverred Trueskip. I definitely prefer the consistency of Foreverred, especially since it allows for even more difficult levels. Though, with the exclusion of Etherealization, it becomes that both Trueskips are technically easier than Gemskip in base Celeste Classic.
Also, I did end up buying the full Celeste on Steam. Apparently, the Farewell chapter is only considered "red intermediate" difficulty, so I have a long way to go.