I think we should improve the documentation at https://www.mongodb.com/docs/v6.0/release-notes/6.0-compatibility/#legacy-opcodes-removed to mention the getLastError command. This command was used by old drivers in conjunction with the legacy op codes for doing writes (OP_INSERT, OP_UPDATE, and OP_DELETE). As part of removing support for the legacy op codes in 5.1, we removed support for getLastError as well. It's worth mentioning this explicitly in the release notes.

Note that we did already document the removal of getLastError elsewhere, e.g. here: https://www.mongodb.com/docs/v6.0/reference/command/getLastError/