I’m on Release Preview ring now and i want to go back to slow ring (or fast in the future), and the only thing that is stopping me is the incompatibility of Avast with Windows 10’s latest previews. Was this fixed?
What latest release preview ?
The win10 anniversary update version, build 14393 is essentially the stable release and that appears to be working fine with avast.
For example slow ring Insider build 14965.
Insider build are not compatible with any decent av.
I’m sad 
. I was missing slow and fast rings and asked because i don’t have the time to try to upgrade builds and then go back to stable if it doesn’t work.
The latest fast ring insider build (14986) works fine with Avast now - its the first one in a while that has!
Good to know that, will try updating to it. 
Quote from https://support.avast.com/support/tickets/563093 !!:
Thank you for responding. Our developers didn’t confirm this problem as fixed, but it is possible that this particular preview version will work with Avast.
Best Regards,
Prokop
The Avast Support Team
You can’t really expect any anti-virus excluding Windows Defender to be compatible with Windows 10 insider builds.