My bet is apple will drop support when intel drop support, so probably the last release of macOS that supports intel will be the one where the 9900 series is EOL in the following year.
The next on the chopping block would Coffee Lake, featured in most of the remaining Intel Mac models. I see little reason for Apple to continue to support Intel CPUs which the manufacturer no longer supports, particularly for a lame duck architecture.
I often hear the argument that customers who purchased an Intel Mac mini or Mac Pro earlier this year aren't getting proper long-term support from Apple, but these folks had to know what they were getting into when buying one. Also, just because you spend a lot on a Mac Pro, doesn't mean that you deserve longer support than any other customer.
A Mac loses support based upon the date it was released, full stop. Deprecate x86, priority one, all other priorities rescinded. That's why I always replace my old Macs with a day one release, in order to get the maximum useful lifespan out of it.
Yeah, the number of Hackintosh users out there is pretty small and there is enough pain running one that I'd say running one is as much an entry drug (this would be great if it everything actually worked!)
I'm living proof of that. After Steve Jobs announced the switch to Intel, I put OS X on my custom PC, back in the day, before "Hackintosh" was coined. I got to play around with this strange niche operating system, and bought my first Mac mini a few months later. Had the Mac stayed on PowerPC, then I would have never switched, because I couldn't have played around with OS X.
Oddly enough, I bought a PPC Mac mini, despite knowing about the transition. A mistake I then repeated by replacing it with a 32-bit Core Duo Mac mini. I've been far more deliberative and cautious about what Macs I plan to buy, now that I understand Apple's methodology better. Despite historical patterns, you'd need a degree one step above Kremlinology to begin to divine the fruit company's plans.