XMS version 10.13 supports Apple APNS http/2 automatically
XenMobile Server XMS use Apple APNS for device push notification and based on the APNS with legacy binary protocol but that will be replaced with APNS http/2-based API. Two years ago Apple announced that APNS with the legacy binary protocol would no longer be supported from November 2020 and migrate to Apple APNS http/2-based API.
But on October 9, 2020, Apple extended this deadline to March 31, 2021, giving you additional time to prepare for this change. See updated announcement here.
This also means that you now do have more time to check your Citrix Endpoint Management (CEM), formerly XenMobile Server (XMS), whether this already supports the Apple APNS http/2-based API.