In the vast majority of cases any package would be available on ARM.
One of the requirements for a package to be accepted into a Debian release is that it has to compile on all of the Debian architectures, not just x86/x64. This includes ARM.
So, this will pretty much guarantee that the vast majority of Ubuntu's packages will at least be able to be compiled on ARM.
That said, Ubuntu is not Debian and may have a slightly less strict policy regarding inclusion of package that fail to build on ARM. While most of their code is inherited from Debian, an increasing amount of it is Ubuntu-specific.
Ubuntu devs do keep track of build failures per architecture, and this includes ARMHF and ARM64.
They also treat failure to build on ARM as a bug.