I'm using SpenceKonde's ATTinyCore libraries, and I'm getting an error when I try to compile a bare minimum sketch that only includes the Servo.h library. The errors are not there when I compile for something official like an Uno.
It is currently not supported. There is an open issue to fix this - it's one of the issues I have really wanted to sort out for a very long time - it's somewhat tricky to achieve, because working with the weird timer on the x5 and x61 series tinies is somewhat challenging.
There are some other (non-core-associated) libraries that provide timer support on the x5, though.
My goal as with the Unified Wire/SPI libraries is to offer an identical API for all the tinies - I've done a much worse job than I had always hoped here.