"The Irish speaking area."
Ae is a relatively uncommon Irish digraph that’s treated as a single broad vowel for the purposes of that rule.
Just out of interest, this is because Ae used to be a separate vowel in Classical Irish.
By “a separate vowel”, do you mean that ae had been written as æ ? Or that the same sound had a different spelling in Classical Irish, but became spelled ae in (Early?) Modern Irish?
In Classical Irish "Ae" was used to denote a separate vowel that no longer exists in the language, i.e. it was not pronounced as "é".
So Gael did not rhyme with scéil (genitive of scéal).
Is the Classical ae sound identifiable now, or is it only known that it wasn’t é ?
Yes it is identifiable. I assume you know IPA given your previous posts:
é = /e:/
ae = /ə:/ (in Classical Irish)
How man other exceptions are there now? This is contrary to how I learned Irish spelling (as below)
This isn't "an exception" - ae should be treated as a single broad "vowel". Unless you can find examples where a consonant after ae becomes slender, then its not an exception, just a fairly rare combination of letters. Leathan le leathan doesn't apply within compound words either, and they are far more common than the ae digraph.