It's mostly embedded / microcontroller stuff. Things that you would use something like SDCC or a vendor toolchain for. Things like the 8051, stm8, PIC or oddball things like the 4 cent Padauk micros everyone was raving about a few years ago. 8051 especially still seems to come up from time to time in things like the ch554 usb controller, or some NRF 2.4ghz wireless chips.

Those don’t really support C in any real stretch, talking about general experience with microcontrollers and closed vendor toolchains; it’s a frozen dialect of C from decades ago which isn’t what people think of when they say C (usually people mean at least the 26 year old C99 standard but these often at best support C89 or even come with their own limitations)