are there really MCU's out there that have 1V8 interface levels? With what do they typically interact?

I recall using a XILINX CoolRunner-II, about a decade ago, that wasn't 5-volt tolerant, though its predecessor, the XCR3 family was. I'm actually still using CPLD's from the '90's, simply because they do tolerate and in some cases "speak" 5V0, though I've had little difficulty getting those CR-II's to drive HCMOS registers, etc when they run their I/O blocks from 3V3. You're right, though, in that none of the mfg's will claim their low-voltage, i.e. 1V8 or 2V5 parts will drive to 5 volts, but most of the external stuff that will interact with TTL will happily tolerate 3V3.

2V5 ... well, not so much.

I found the Spartan-II series quite versatile, first of all because they have the ability to operate at nearly any I/O voltage under 5V0, AND they have internal tristate busses, though that seldom matters. Another interesting feature is that there are separate supply connections to various I/O blocks, so some of the I/O can be at one voltage and another can be at another. That has good likelihood of being useful. The original Spartan-II's were 5-volt tolerant, but the IIE series, which was more popular, were not. Too bad ... <sigh> ...

I remember that one ARM we used back in the day (about a decade back) wanted 1V8, 2V0, 2V5, and 3V3. I'm hoping they've figured out how to do with fewer supplies by now.

RE