What happened to this? In previous versions, recorded notes and the quantisation of the recording (if active) have been two undoable steps: undo once de-quantised the recording, undo twice removed the recording (clip).
What happens now (9.0.3) is that there are still two undo steps associated with recording a new session clip, but they're both called "undo slot recording", and the last one doesn't seem to do anything, whereas the first does what you'd expect.
I really liked being able to quickly undo quantisation, because it's faster than recording un-quantised and then quantise after the fact. Also, I keep record quantisation on by default.