I know of companies using multiple logical dbs in same mysql instance ( like create database db1, create database db2 etc) for parallel replication since 5.6.
In 5.7, new feature was added to be able to parallelize replication within one logical db.
We tried it once more than a year ago with the following settings
slave_parallel_type = LOGICAL_CLOCK slave_parallel_workers = 20
We saw replication broke on slave with this weird error:
Last_Errno: 1755 Last_Error: Cannot execute the current event group in the parallel mode. Encountered event Gtid, relay-log name ./db4030a-relay-bin.009066, position 73699 which prevents execution of this event group in parallel mode. Reason: The master event is logically timestamped incorrectly..
Since that was a new use case anyway, at that time we decided to use multiple logical dbs for parallelizing replication rather than debug the issue.
Now for our main legacy use case still on 5.6, as we migrate to 8.0, we are wondering whether to use parallel replication with just one logical db or try to split the tables into logical dbs.
If you have a large mission critical use case with hundreds of diverse tables, I would like to know if parallel replication with one logical db is causing any issues for you.