OsmoSGSN LLC implementation doesn't refuse ABM
the LLC layer has a (specified) Asynchronous Balanced Mode (ABM), which is very much like LAPDm. I don't think it's used much in production. In any case, the OsmoSGSN LLC code implements a very small fraction of that, without properly handling it.
The better option is to outright refuse any etablishment of ABM, and hence inform the peer (MS) that ABM is not supported by sending a DM frame. This is also what the spec states in Section 22.214.171.124 of 3GPP TS 04.64:
The DM unnumbered response shall be used by an LLE to report to its peer that the LLE is in a state such that ABM operation cannot be performed. An LLE shall transmit a DM response to any valid command received that it cannot action.
The fix to OsmoSGSN is in https://gerrit.osmocom.org/#/c/osmo-sgsn/+/13767/ and the related ttcn3 test is in https://gerrit.osmocom.org/#/c/osmo-ttcn3-hacks/+/13766/