Sure Bootstrap 5 will be supported, I’m hoping to offer a framework selection for V4, V5 etc.
If Blocs v4 comes out before BS5 is out of Beta it may have some kind of warning popup if you choose to build a site with it, until it’s a public release.
The Blocs project migrate function from v4 to v5 may also come once Bootstrap v5 has fully shipped.
The first alpha for BS5 just dropped, so I’ll most likely start implementing parts next week.