There's nothing for you to do, as far as I can tell...
First...
>> We have an Entra ID enterprise application called iOS accounts... Rumor has it that this enterprise application is required for iOS devices to connect to exchange online using the native mail app.
Check your rumors... I have used Mail on iOS for years, with multiple Exchange accounts, and I've never heard of this app, so it's clearly not an absolute requirement, unless it's required by your own corporate policies.
Secondly, Mail on iOS doesn't use EWS, it uses EAS (Exchange ActiveSync) which doesn't appear to be affected by this migration.
Even if Mail was using EWS, as a core application, you'd have to assume that Apple would update the app to manage the transition - you're not the only Exchange user on iOS, and the impending change has been (reasonably) well communicated. Apple aren't going to throw all those users under the bus.
With 5 months until the transition, there's still time for custom applications to get updated, but I wouldn't be worried about the core applications - at least for users on supported iOS and MacOS versions (if you're running out-of-date systems that are not actively supported, then there's a chance you'll have trouble, but I can't comment on that).
