Hello Mike,
thanks for your interest in getting pinta updated in Debian. The other day I noticed as well that pinta is outdated in Debian and it does not look like there is a simple way forward, unfortunately.
Pinta has moved to the dotnet runtime which is not packaged in Debian.
Many years ago I worked on getting the dotnet core runtime into Debian (just as I did to get Mono into Debian back then) but I hit hard problems that prevented it.
The dotnet runtime could not be build from source which is not compliant with the
DFSG. Microsoft had shown interest and I have advised them on how to make dotnet DFSG compliant so it could be included in Debian and Ubuntu, but it was clear it won't happen overnight as building cleanly from source (bootstrapping a runtime) isn't trivial and needed a major effort on the upstream side. Later this effort deepened between Microsoft and Redhat to make dotnet buildable from source, which is great.
I am not sure if you can build dotnet from source at this point, maybe it is possible by now. I could never find time to follow-up on this as I started to work for demanding startups that leave little to no spare time. If you are interested to get dotnet into Debian I am still available for mentoring and going in the right direction, I would like to see dotnet packaged still, it is a fantastic software development platform. The #debian-cli IRC channel on OFTC is the place where the Mono and .NET friends hang out, so feel invited to join us.
Best regards,