I left shortly after the IBM acquisition was announced, and I'm so happy I did.
The brain drain happened shortly after the announcement. To me it seems like all the driven, ultra-sharp people started leaving in 2019 and in force in 2020 and early 2021. The people I've interacted with Red Hat since 2020 were not anywhere near as sharp as the folks I worked with prior.
EDIT: I was probably too strong on my statement that "all the driven, ultra sharp people". I know that I have a lot of ultra sharp, smart, engaged friends still there. What I meant to say was that I started to see the drain late 2019.