The Research and Innovation office (RIO) has advice and guidance on REF2021 on their pages.
Final guidance from HEFCE on REF2021 is not yet available, but will be uploaded here in due course. The official REF website is the best source of information for what has been decided to date.
Section 37 and 38 deal with open access issues. Annexe B is also useful.
Final REF Guidance issued January 2019