B2 has now S3 compatible API

It seemed valid to me, otherwise I wouldn’t have said it. Looking at the “2020 week 17” results, I see:

70084 Backblaze
67045 S3 compatible

The numbers look valid and support the concern about pushing a more-used option onto a less-used.

S3 is pretty clearly going to stick around, in spite of occasional issues with not-quite-compatible “S3”.
One good/bad thing about the S3 libraries is they’re third-party, so Duplicati can’t really change them.
I hope someone volunteers to try Backblaze S3 while they’re still in Beta, and maybe more receptive.

Exactly, although sometimes a storage vendor forces the issue. Amazon Cloud Drive caused 2.0.4.23 which was done just to issue a warning that it was going away. Original OneDrive API end was messy.