In Eclipse IDE, the Undo and Redo actions are disabled (greyed out in menu and toolbar, shortcuts ctrl-z, shift-ctrl-z don't work). My Eclipse version is 2023-09 (4.29.0).
I noticed that the actions had stopped working when I was editing a Python file (using PyDev, but I doubt that this was specific to this setup - at least this similar post encountered a very similar situation for a different setup).
If there was a specific trigger for the problem, I didn't notice... Neither restarting Eclipse, nor restarting the computer, did work (as suggested here)
This is basically an update of this question, which is so old, that it may seem out of date. It is not, though.
Here is the fix, that worked in my case:
Go to Preferences→General→Editors→Text Editors. Change history size. Re-start Eclipse.
(I changed it from 200 to 0 and then to 1000, but maybe it would have been already enough to touch it once)