I don't think it's abandonware, per se, so much as that gRPC deliberately stays way behind the times with respect to Java language versions. So that they can support enterprise users who are conservative about Java upgrades. I don't know where the Java implementation is now, but when I was using it ca. 2019, it was still officially targeting Java 7. This was after the end of public updates, but still well within Oracle's paid support period for that version.

Java 7 support is now completely over, so I'm guessing now they're targeting Java 8. This does create an annoyance for Java 9 and later users, and requires annoying workarounds for them. I don't see this as a maintainability problem, so much as a culture clash: gRPC is doing things the Google way; other people don't want to do it the Google way.

All that said, I don't think other people are wrong. This is exactly the kind of thing I was complaining about - a lot of gRPC's potential (at least for my purposes) is undermined by its status as a big complicated enterprisey Google monoculture project.