Salesforce released a new Lightning Threading approach, which, according to their documentation (https://help.salesforce.com/s/articleView?id=000389648&type=1), is: "In this new threading behavior, incoming emails aren’t matched using Ref ID strings. Instead, they’re matched using a secure token in the email subject or body. When no match is found using token-based threading, Email-to-Case uses header-based threading and checks metadata in email headers." Salesforce expects everyone to fully migrate to this new Lightning Threading behavior by Spring '25 with the “Disable Ref ID and Transition to New Email Threading Behavior” release update. It replaces a release update that was first made available in Winter ’21 to transition from Ref ID threading to header-based threading.