The PL/java “add-on” is not supported directly by Postgres.
With a few exceptions (Perl, Python) this is how the community handles all "add-ons". A great number of features available to you are available as third party extensions and not part of core. Zombodb, Timescale and Citus are good examples.
In short, don't worry about whether or not it is "official".
JD
I would like to ask if there is a plan to include it as an official procedural language. If so, when?