Hubzilla is not OStatus, it's Zot and later Zot6. Hubzilla have optional plugins for ActivityPub and OStatus.
The Diaspora Network (a.k.a. The Federation) has a plan to add ActivityPub support. When it started 6-7 years ago, it had OStatus support. They decided to drop it because it was a P.I.T.A. t maintain and taking away their time from their own "diaspora protocol" (don't confuse it with the software, there are other software connected to the Diaspora Network/Protocol).
The only one left still using the obsolete OStatus protocol is GNU Social. OStatus was developed mainly by the main develop behind GNU Social a.k.a. StatusNet, Laconica, Chris Evans. He left and started Pump.io. While developing Pump.io, the beginnings of ActivityPub formed. He left Pump.io to concentrate on ActivityPub.
Believe it or not, Pump.io did not even bother to implement OStatus. They also closed/dropped the plan in favor of ActivityPub which still has zero progress.
GNU Social fans are developing an ActivityPub plugin, one is already in testing, but it is taking far too long because there simply is no interest within their community. (Like in Diaspora software side, too long, no interest, just a side-project with the least priority.)
In other words, OStatus is dead, obsolete, forget about it. It is time to move on. ActivityPub isn't perfect but everyone rallied behind it and not OStatus.
Other developers and projects and server admins are dropping OStatus support and/or plugins.
We just have to wait for Friendica to make their AP implementation stable and we're completely done with OStatus. If GNU Social wants to stay isolated, it's their choice.
Heck, even Socialhome is developing its implementation of AP in their code, Socialhome is mainly Diaspora Network/Protocol. So, Friendica isn't alone in this. Friendica is mainly DFRN with Diaspora Network/Protocol and OStatus, soon ActivityPub too. Will Friendica drop OStatus? I hope so. It's the only way to tell GNU Social fans to migrate to AP.