Is Cubase 9 compatible with the older Yosemite 10.10.5 operating systems ?
Its not supported , it may work but any issues are yours to fix as it were.
It works completely without issue in 10.10.5. if you want to upgrade to Yosemite, you’ll be fine.
For me it did not install the app on 10.10.5, only the contents. someone know the answer?
I try C9 under yosemite and sierra ,
With yosemite i had a lot of blacklisted plugins , but under sierra only 2 blacklisted !
It seems C9 work quickly and better under sierra…
Thank you for this question.
And for the replies.
I’m on 10.10.5 and because upgrading an OS is fraught with possible issues, I try to avoid if possible.
Was looking forward to upgrading to Cubase v9 after reading/seeing the promo & you’ve given me confidence to go ahead.
At least I have v8.5, v8, v7.5, v7 to fall back to if/when necessary 
Will try to remember to post my experience here on a later date!
Neil
Works well with OS 10.9. I`m on Cubase Pro 9.0.1
Best regards
As ‘promised’, update on Cubase 9.0.1 on 10.10.5.
No issues found so far. Enjoying the changed workspace. Although a bit amazed I can’t seem to show the time signature on any toolbar. Only the good old Transport Panel.
Neil