Recently, I learned that PostgreSQL can integrate with many languages through plugins, such as PL/Python, pgrx (Rust), pgzx (Zig), and so on. I wonder if anyone is planning to write one for Java or C#, lol.
There has been PL/Java for a long time: https://tada.github.io/pljava/ and PL/Perl and others. The projects you mention are largely building on well-established interfaces in Postgres.
Not that I'm aware of but any plugin system that expects dynamically linked binaries integrating through C API can be targeted by .NET NativeAOT (e.g. OBS plugins can be written in C# without manually dealing with hosting the runtime).