Again, good point. I don't know either if aerial recon has ever been a casus belli ; i'm too lazy to research it but tbh i wouldn't be surprised if it happened considering the ridiculously smaller things countries have gone to war for.
But then again, you said it yourself : recon planes, while evasive, were not invisible. We're talking about making them undetectable (to a certain point), which is an entirely different issue from what you're trying to back up with historical examples. While i see what you're trying to prove, the examples are the wrong ones in any case.
The real gripe I have with this suggestion is more regarding its use against AI :
I don't think there is a need for automatic war against a player as soon as his plane crosses my border. Of course , if that happens, I'll be very cautious, but I probably won't go to war for that.
The problem is, however, that you can't make an AI country to react the same way. You can't implement an AI that goes through the same thought process of suspicion as a player goes ; yet there is a need for an adequate reaction, as i'm sure you'll agree. It's just idiotic to have the AI country let you patrol his skies at your leisure.
The only possible correction to that, then, is to make it declare war automatically ; there is no other minimal alternative that'll be good enough and still simulate the country's being wary of your constant spying over him. So as you can see, while in reality air recon was a common (and tolerated as you seem to think ? Don't know but let's assume it) practice, the need for an adequate automatic response creates the need for an automatic-war-setting from a realism and developer's standpoint.
If you want a very fast plane that can do recon fast and then retreat before being caught, I guess there's the rocket plane for that.
As a whole, I do agree air recon should not (necessarily) be a cause of war. The planes should be definitely (tried to) be shot down though. And in any case it needs to be there regarding AI countries or otherwise there'll be a complete lack of possible (and necessary !) reaction on their part.