I'm passing these questions on (that I didn't know the answers too offhand) for a friend at USC...
Thanks for any help!
If they follow the usual schedule (which I don't know because of COVID), the 2020 data products (2020 1-year and 2016-2020 5-yr) should be using 2020 Census geography vintages. They don't re-release old data with the updated geography boundaries because small changes in borders might make some respondents identifiable.
More info: https://www.census.gov/content/dam/Census/library/publications/2020/acs/acs_geography_handbook_2020.pdf#page=18
But this document says: "More specifically, ACS data generally reflect the geographic boundaries of legal areas as of January 1 of the estimate year." Were the new 2020 geographies already in use by Jan. 1, 2020?