With SCOM 2019 UR2 change tracking is added. That allows you to create reports to find changes made to overrides. See this document for more information: https://learn.microsoft.com/en-us/system-center/scom/management-pack-change-tracking?view=sc-om-2019#overrides-tracking
Changes to subscriptions are however not changes to management packs and are therefore not covered by change tracking. There is also not a eventlog that stores this information. I'm not aware of a solution that can give you this information.