I mean, not being able to delete some files because they're read-only and what not doesn't mean it's less open. You can't do that on an immutable distribution either but I definitely still consider them "open".

At postmarketOS we're planning to also offer an immutable option in the future. We'll always keep a mutable option around for people that want it but by default we'll ship the immutable variant to our users because we can guarantee stability and safe updates way more. Then you won't be able to just delete system files either.