3GPP 24.501 v20.0.0 — the document's own text
8.2.19.16 Rejected NSSAI
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
The network may include this IE to inform the UE of one or more S-NSSAIs that were previously sent to the UE in the allowed NSSAI or the pending NSSAI, but are now considered rejected by the network.