Mistwell said:
"for wizards..., Extra Spell is generally used to learn a spell that the character lacks access to and would unable to research"
...is generally used...
These words alone strongly imply, that the words following them do not provide extra rules, but an explanation to the previously stated rules (that the feat allows you to learn an extra spell). It's an attempt to explain why this feat does not totally suck for wizards (which it sure does, even with the explanation). And nothing else.
It sure seems possible that extra spell is a specific exception to the general rule that a wizard cannot learn a spell from a prohibit school.
That still (

) seems to be a very lenient interpretation of the text to me, especially since the feat description doesn't say anything about that. And some rather drastic change in the basic rules surely would have been stated in a little more direct fashion.
Feats (or any rules for that matter), have to state what they do, not hint at. If they do not state, that you can suddenly learn spells not on your class spell list, then you can't do that.
It surely would have been better to use stronger terms than 'not available to', 'lacks access to'.
They should have stated explicitly, that classes can only learn spells from their class spell list.
They should have stated explicitly, that spells from prohibited schools are removed from the class spell list.
They should have stated explicitly, that Extra Spell allows you to learn one more spell from your class spell list.
They didn't, these things only exist in the meta framework behind the rules text.
They didn't, because this general rule, that you can only learn and cast spells from your class spell list, is something they probably consider so obvious, that they do not state it anywhere explicitly (except the casting part, that is, which
is stated in the PHB).
And once more... a general rule is still to be followed, unless it is in conflict with a specific rule, in which case the specific rule should take precedence.
This feat works just fine without having it break the general rule about learning spells, and therefore this rule is still in play, since it is not explicitly disabled.
But then, I've had this exact same debate, with Thanee in fact, before.
Bye
Thanee