Is Librarian 4.3 compatible with z/OS 2.1?
The z/OS Compatibility matrix on CA Support Online only shows Librarian 4.4.
Yes it is.
If upgrading from z/OS 1.9 or later, no additional maintenance is required.
If upgrading from z/OS 1.8 or earlier, need to apply QO89534.
*** Be advised ***
Librarian 4.3 is out of support and you need to upgrade to Librarian 4.4 at your earliest opportunity.