Ancient version like 15 or 16? I have recently encountered a PostgreSQL 12 instance.
Once the database runs you are inclined to let it keep on running until you must do an upgrade (Bugfixes, EOS) . Upgrading for new features/ improved performance is nice but can be a pain and is prone to cause downtime. PostgreSQLs upgrade tolling story is also a bit painful but it has proven to be an awesome way to semi-regularly test my backup infrastructure.