I would say that UBports questions are on topic. It is not an Ubuntu project in the way Kubuntu or Lubuntu might be, but Canonical wrote the vast majority of software that makes up what UBports is, still owns all that Copyright, and is allowing UBports to use the Ubuntu trademark.
The requirements for being an official flavor are untenable for a mobile phone distribution like UBports. Even for the 3-4 years while Canonical was actively developing Ubuntu on phones, it never was able to meet all the requirements of being an official flavor of Ubuntu.
Even if UBports managed to build an ISO install for traditional PCs, and comply with all requirements for becoming an official flavor, the fact remains that the phone builds never could achieve this, simply because they cannot use the Ubuntu kernels. This would mean UBports (and Ubuntu for phones/tablets, aka ubuntu-touch) will never be on topic here, and never was (despite it being so simply because it was an official thing from Canonical).
IMO, the rules for being on topic are going to have to change for situations like this. I would also argue that Plasma Mobile questions should be on topic, as it's being developed by Kubuntu and technically should be considered a derivative of Ubuntu for phones (ubuntu-touch, ubports).