To answer your most recent questions:
> That'd affect 10.3 or 10.4 as well, depending on what's considered latest.
Yes.
> it'd be interesting to understand the rationale for that policy
It basically boils down to wanting to support as many configurations as we can with finite resources. We can't support building on everything so, we do what makes the most sense. If a distro has chosen to move to only offering 10.3, we take that as a reasonable baseline, and we'll do 10.3+ there.
> Does it make sense to build the same versions? As they should be already available..
It makes perfect sense. We work with distributions to fix bugs. It'd be hard to do that if we weren't building and testing what they are shipping/supporting.
> on Sid
Note this applies to Buster too.
> You'll be fighting with Apt to not auto-upgrade your packages to the 10.3 packages in the Sid repositories.
Not really, as the version is part of the package name. apt install mariadb-server-10.1 should work just fine, even if a mariadb-server-10.3 is available too.