TBH, the current function name pg_last_wal_receive_lsn() is confusing, and introducing a new one pg_last_wal_receive_unflushed_lsn() doesn't make it better :(
What about actually adding TWO new functions, pg_last_wal_write_lsn() and pg_last_wal_flush_lsn()?
These names are more aligned with column names in pg_stat_replication view and speak for themselves.
And, we can keep pg_last_wal_receive_lsn() as an alias of pg_last_wal_flush_lsn() for backward compatibility.