I have been developing a theme with deep integration for BuddyPress. At one point I was developing on XAMPP with WordPress 5.XX version. I have recently started development on XAMPP with ClassicPress latest version.
I am using the latest version of BuddyPress, Stable tag: 9.1.1. Although some block elements have been introduced to BuddyPress, I have seen no issues, alerts, or warnings with ClassicPress compatibility. Thus far BuddyPress 9.1.1 is still listed as requiring WP 4.9.
I do not use the default BuddyPress templates, Legacy or Nouveau. I have created a separate template pack for this theme, but when switching between for testing, all seems to run as expected.
I just thought I would put this info out there for anyone considering BuddyPress with ClassicPress. I don’t know how compatibility will be for BuddyPress going forward, but traditionally BP development has been slow paced, not to mention, existing sites that use BP generally undergo quite a bit of customization and It would not be wise for BP to bump the min WP version to anything greater than what it is now,
I suppose if worst case scenario in the future, current BP version could be forked, stripped of block elements, and dubbed ClassicBuddy.