PCI-SIG does not publish official or “standard” channel models; however, the Electrical Workgroup (EWG) does post example channel models. For PCIe 5.0 specification, the reference package models are posted here: https://members.pcisig.com/wg/PCIe-Electrical/document/folder/885.
You can also find example pad-to-pad channel models shared by a few member companies during the specification development by searching *.s24p in the following folder https://members.pcisig.com/wg/PCIe-Electrical/document.