It can be. We still running PG11 and 13 in production. Not obligated to upgrade - especially when the software is rock solid.
If I was running PG 11 in production, I'd be looking to upgrade sooner rather than later. Being on older-but-stable versions is good risk management, being on end-of-life version is not.
Not quite yet https://www.postgresql.org/support/versioning/ - the final release will be November 9, 2023.
Edit: quick check shows we also have PG9 and PG10 in some Live roles too.