Not anymore, well actually it started 5 years ago as AVR, there custom branches for esp, but I just rearchitected it all into one, so it supports a bunch of platforms. See wiki for docs on the V2 which is master branch and shows up in Library Manager.
@AdySan I gave it a try (compiled only), it doesn't currently match any of the Arm defines I have. First though, you may want to give them feedback that they are missing yield () function. Arduino added this "optional" api, important for all the 32bit chips due to core requirements. Even most AVRs use it.
That link https://www.arduino.cc/en/Reference/SchedulerYield is probably best. I know its optional as Gemma (AtTiny) doesn't support or have it; but almost everything else does. No, that will block, yield() will return if there is nothing that needs to be done by the core.