asrock hasn't been a subsidiary of ASUS for 20+ years, but its complicated because they're owned by Pegasus. Pegasus was created in a restructering of AsusTek, so its hard to say whether or not asus still owns them or not but at arms length. They're both companies that were spun off but the corporate structure is complicated.
and pegatron owns asrock, and asus is still the largest single shareholder in pegatron. so does asus own asrock? not entirely, not directly, but sort of.
Its sort of in their wikipedia. Pegasus was the production arm of asustek, was spun off into their own company, but asus is still the largest single shareholder but doesn't have a majority ownership. Pegasus owns Asrock.
(Just a random fun fact, not implying its product quality now)
*EDIT: I mean ASUS funded it and owned it by the time. I'm not sure if "subsidiary" is the correct legal term here.