Yes, you're correct, that deprecated APIs often break, but here is a big caveat, especially for OS APIs:
Deprecation in itself, doesn't warrant breakage or removal.
It normally means that you shouldn't use this API for new stuff. E.g. look at Carbon: It's long deprecated, but as of El Capitan it still runs.
That the old audio API (which by the way?) is broken in the Beta, only means that it's broken at the moment.
Because as long as Apple doesn't clearly state, that it wants to remove this particular API, they will probably fix it.
I can't think of a reason, why Apple would wantonly break things for Logic 9, even if they don't sell it anymore. Even FCP 7 still runs on El Capitan out of the box.
As an aside: Logic X is in my opinion the superior product, so you should really consider buying it. I have worked with both version, so I tell that I never looked back to using Logic 9 again.