wal retention in pg_wal and wal archiving are different things. The OP got problems cause he deliberately went on to delete files in pg_wal which means he used the wrong method to address a wrong situation.
I don't think we are disagreeing. My point is that WAL will be retained in pg_wal if it is still needed by a replication slot, and any one of those WAL files will not be archived and recycled as long as it no longer needed by a replication slot. So yes, they are different things, but they are related in that sense.
It sounded to me like he had an old slot in place pointing to old WAL files, which the cron job removed, thus leading to his error.
Sure, he can recover from this by moving those WAL files back into pg_wal. But my point is simply that he should rely on WAL archiving rather than archiving via cron because Postgres handles knowing whether or not a replication slot still needs a WAL file.
