Doesn't seem to do anything on D 10.2.3

Also seeing this. Drupal 10.2.6.

There is no enforcement of anything. Nodes can be saved without log messages.

I noticed today, that upon initial creation of a node, there is no requirement to complete the revision log.
Later, found that any edit, revision change to that same node, DOES require a log message.

So, I think this is one of those "did not meet the users expectation." type bug. Think you should require a log message initially, from the very get-go.