Thank you for the reply.
I tried adding a file from the cloud drive, and received this error: "Cloud
drive file is invalid or not found." However I can open it in Google Sheets
and edit it just fine.
If I can get the workaround to work, it could suffice temporarily, but the
real benefit from linking a Google Sheet is that the map is automatically
updated when any changes are made to the Sheet. I just don't have time to
do manual updates whenever there's a change (there will be a lot).
I'm not sure if this is an ArcGIS Online problem, or a Google Sheets
problem. I just know that on Jan. 16 all my sheets were linking to
webmaps just fine, and as of today (Jan. 30) they are not, and I changed
nothing in between there.