Hi
> We currently running Ubuntu 18.04 and Postgres 10.23.
That's what I thought. Correct, postgresql-15-postgis-3 was not packaged for for ubuntu 18.04.
I don't know the backstory, maybe because it was only a few months from the postgresql 15 release to the EOL of this ubuntu release, or there were issues with the build. But there are no packages and now they are definitely not expected to appear.
regards, Sergei