Which of the prequels are canon to Persona 5 - Strikers?
Persona 5 Strikers is a sequel to Persona 5. There are no references to the additional characters or new palace/semester that Royal introduces in Strikers. This seems to be more related to the development timelines than a choice in canon, as both Royal and Strikers were simultaneously under development.
There are a few sources that support this, including: https://attackofthefanboy.com/guides/is-persona-5-strikers-a-sequel/