You were probably not keeping the app updated before it was removed from the store.
So now, being a very old app, it is probably no longer compatible with recent iOS version updates.
I currently have iOS 16.1.2 (20B110) with MT4 build 1351.2 and MT5 build 3400.4 on it and apps are working properly.
EDIT: I've noticed that there is prompt for updating iOS to 16.2. I will run the update and see if the apps will work on it or not.
I've now updated my iPhone 11 to iOS 12.2 (20C65) and both MT4 build 1351.2 and MT5 build 3400.4 are still working without issue.
If you wish to continue to receive push notifications without the MT4/5 apps,the consider installing MQL5 Channels app ... FYI: Push Notifications on iOS devices — "MQL5 Channels" app.
If your broker offers the MT5-Webterminal (ask him) you can use this instead of the app, (https://web.metatrader.app/terminal)
I've now updated my iPhone 11 to iOS 12.2 (20C65) and both MT4 build 1351.2 and MT5 build 3400.4 are still working without issue.
Thanks @Fernando Carreiro for your help. My MT4 build was on 1328, so a very old version.
I've managed to update my app by going to the:
App store - profile - and then purchased section. It then allows you to update the app even thought it's removed from the app store.
The app to the latest version.
I think this will still work even if you change your phone, you can login and install the app as long as you've downloaded it previously....
For any users wondering
If your broker offers the MT5-Webterminal (ask him) you can use this instead of the app, (https://web.metatrader.app/terminal)
I have MT5 for iOS on build 5.0.3441 (seems much newer than 3400, , and I received this prompt to update last night.
as a troubleshooting step, I updated to iOS 16.2, did not help out.
Any ideas?

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Hello Mql5. Are there any IPhone users getting this message today?
Unfortunately, your program is outdated and no longer supported.
Please update the application to continue working on it.
Of course MT4 isn't on the appstore so I can't update it.
any ideas? thanks.