As I understand it, the Digital Markets Act came in to force in May of 2023. Apple was designated as a “gatekeeper” due to their restrictions on app installation this month (September 2023), and now has 6 months to comply with the DMA’s rules.
The rules on messaging services for interoperability have been given longer periods to reach compliance.
I’m not sure if there any sort of appeals process that can be used to stall or reverse implementing the rules. Like a lot of EU acts, the DMA is rather complicated in the details.