I have done it both ways since there is no clear guidance. If I have evidence the client has satisfied all requirements for PPPL to be fully forgiven by 12/31/2020, and they do not submit the application or receive forgiveness until 2021, I let the client decide but we generally agree to follow matching principle. It is also my belief it satisfies accrual rules for recording as income/expense in year the SBA's forgiveness criteria are satisfied, since that is ultimately what determines forgiveness approval. Well, unless a client never submits a forgiveness application, at all, but to date that act has been known before financial statement issuance.
EIDL Advance and PPPL have been recorded as government grant income under "other revenue."
I'll change it if necessary when I see clarified guidance, which has not yet been issued (as far as I can find).