It turns out that ALTER ONLINE TABLE is not the same online alter as online alter in MDEV-16329, so the example in the description doesn't exactly relate to it (you can't do ALTER ONLINE TABLE ... RENAME but you can do ALTER ... RENAME .. ALGORITHM=COPY, LOCK=NONE); but the question of non-detailed error messages remain the same, regardless the example.

I think we do need better diagnostics. The error message for 1845 ER_ALTER_OPERATION_NOT_SUPPORTED, as in the description, is at least still correct, even if not very informative ("LOCK=NONE is not supported for this operation. Try ..."), but the more generic 1846 ER_ALTER_OPERATION_NOT_SUPPORTED_REASON is now plain wrong: "LOCK=NONE is not supported. Reason: COPY algorithm requires a lock. Try ...".

That is, instead of referring to a specific operation, as 1845 does, this one claims that LOCK=NONE is not supported for the COPY algorithm at all, which is of course not true anymore.

Besides, it can be very confusing, e.g. when you can do ALTER, but cannot do the same ALTER with IGNORE:

MariaDB [test]> alter table t force, algorithm=copy, lock=none;

Query OK, 2 rows affected (0.065 sec)                      

Records: 2  Duplicates: 0  Warnings: 0

MariaDB [test]> alter ignore table t force, algorithm=copy, lock=none;

ERROR 1846 (0A000): LOCK=NONE is not supported. Reason: COPY algorithm requires a lock. Try LOCK=SHARED

dc7136481bf5 looks ok