If the figure was closed, or you clicked before on Save modifications the Undo all modifications option will not work.
Was this done in the Default anatomy? Was it the only change?
If so, you could reset the entire default anatomy with right-click on it, use template, as use the same Default anatomy (ICBM152 2023b is the default for new Protocols)
In a recent commit, editing provided Atlases is not allowed: commit 5778a5e
Thus, a copy of the atlas needs to be created to be editable
In an Advance section of the Scouts tutorial:
https://neuroimage.usc.edu/brainstorm/Tutorials/Scouts#Menu:_Sources