In fact, none pgadmin can be compatible with all versions of the database.
We've done pretty well so far, supporting right back to 7.3, through we don't officially support the much older versions now, but that's mostly due to the amount of testing it requires.
Although earlier versions of PG don't support GRANT ON SEQUENCE,
but new versions support. So I think this adjustment is necessary .
Old versions supported GRANT ON TABLE for sequences iirc, (as sequences are members of pg_class). This change is only necessary if newer versions of PostgreSQL dropped that syntax. It may be desirable, if we no longer support any versions of PostgreSQL that needed the old syntax.
That syntax was (once) intentional, and was not a bug, as earlier versions of PG don't support ALTER SEQUENCE. Did you check the version compatibility?
I'm not sure I understand what you mean. We're not talking about ALTER SEQUENCE. We're talking about GRANT ON SEQUENCE.