The list of classic Mac OS programs which actually spawned preemptive threads was probably in the single digits. Very few of the machines were multi-core, so it didn't matter much.

I'm not sure exactly how many programs used them, but support was fairly widespread in a lot of popular programs because it was the only way for Classic Mac OS to make use of the second processor in multiprocessor machines. Some examples of software that used preemptive threads are Photoshop, Final Cut Pro, After Effects, Premiere, Lightwave, and Cinema 4D (see https://www.macworld.com/article/159778/mp_mac.html ).