The undo stack doesn’t include operations done in the mixier, such as removing or swapping effects on tracks. I found myself several times having to re-load a project and redo some work… unless i’m missing something?
Edit: this also includes other operations such as enabling/disabling tracks, etc…
It is a common request, not implemented and with a huge number of complications that may not be particularly obvious to users. There’s no timeline for it being implemented.