We should document considerations in https://www.mongodb.com/docs/manual/core/replica-set-arbiter/  for sharded clusters with arbiters so that it's very clear what the limitations of arbiters are.

In SERVER-64462  the issue (Changing shard keys errored for replica sets with an arbiter) was caused by unawareness of the fact that:

transactions whose write operations span multiple shards will error and abort if any transaction operation reads from or writes to a shard that contains an arbiter.

The context has been documented in transactions-sharded-clusters we believe it would be helpful to add a similar context in replica-set-arbiter (The first link that shows up by googling "MongoDB arbiters") to bring awareness of the limitation of using arbiters to users.