Prior to 3.6, we used to error in all cases where a replica set name is not included. Obviously this meant you couldn't connect to multiple mongos routers. So we removed this restriction and in doing so made it easier for people to make a mistake of not including the replica set name when connecting to replica set nodes.

mongo "mongodb://localhost:27017,localhost:27018,localhost:27019/"
3.4.19 - FailedToParse: Cannot list multiple servers in URL without 'replicaSet' option
3.6.10 - Connection success
4.0.5 - Connection success

I would like us to surface a warning in this case.