Outdated application drivers that use deprecated opcodes are no longer supported in 5.1+. Applications using outdated drivers will no longer see operations complete after upgrading the cluster to 5.1+.

In the 6.0 docs, we should include a larger warning in the following locations informing users that drivers using legacy opcodes, that were deprecated in 3.6, are now unsupported and recommend to users to update their drivers to a version that uses supported opcodes.

https://www.mongodb.com/docs/v6.0/release-notes/6.0-upgrade-standalone/#check-driver-compatibility
https://www.mongodb.com/docs/v6.0/release-notes/6.0-compatibility/#legacy-opcodes-removed