The startup warning about "Read Concern Majority" is shown when the users have an arbiter, regardless of the number of nodes. If we check the documentation, it is only recommended when the replica set is PSA.
"If you are using a deployment other than a 3-member PSA, you do not need to disable read concern majority."
When the warning was created on
SERVER-37557 it was decided to alert when an arbiter was present, regardless on the replica set configuration:
"the risk exists in any set with an arbiter, since they allow you to accept writes when the majority commit point cannot move forward. We are going to recommend all sets with arbiters use enableMajorityReadConcern:"false"."
So, the warning is not following the documentation, and this could be confusing to the MongoDB users.