I made a short review of technical changes introduced by Piwigo 2.9. As far as I can see, only the update of datatables to version 1.10 could be a problem, but I don't know any public extension that use it.
Do you see any other technical change that could lead an extension compatible with 2.8 to be incompatible with 2.9?
Maybe you can create already the 2.9 version in extension manager? (I can update mine at least)
As soon as we add version 2.9 in the extension manager, beta-testers on 2.9.0RC2 will have no extension available (as long as the extension is not marked as compatible).
Last time, for 2.8, considering it had very little impact, I had automatically marked all extensions compatible with 2.8 if it was compatible with 2.7. I think I will do the same for 2.9, unless we have a good reason to avoid that.