[...](f) refrain from requiring business users or end users to subscribe to or register with any other core platform services identified pursuant to Article 3 or which meets the thresholds in Article 3(2)(b) as a condition to access, sign up or register to any of their core platform services identified pursuant to that Article
"Operating systems" is part of the "core platform services" definition. Locking the use of iOS or sideloading capabilities behind an Apple ID (another "core platform service") would be a violation of the provision.
But that's the European law. Nothing stopping them locking sideloading for US Apple IDs I think? I mean they can't require an EU apple ID in order to access the sideloading feature, but they could deny it to US apple IDs.
[...]gatekeepers shall:
[...](f) refrain from requiring business users or end users to subscribe to or register with any other core platform services identified pursuant to Article 3 or which meets the thresholds in Article 3(2)(b) as a condition to access, sign up or register to any of their core platform services identified pursuant to that Article
"Operating systems" is part of the "core platform services" definition. Locking the use of iOS or sideloading capabilities behind an Apple ID (another "core platform service") would be a violation of the provision.