What's a good metric for when to stop masking?

You didn’t say what part of the world you are in. But for the U.S., Worldometers case counts are no longer accurate due to several reasons. I think all U.S. states now only report cases no better than weekly (and many are now monthly). Additionally, relatively few cases ever get noticed by the various health systems – and so there are scads of subclinical “off the grid” COVID cases that never get captured.

However, you can go to Biobot.io and see wastewater sampling for COVID aggregated from all over the U.S. Concentrate on the “Last 6 weeks” and “Last 6 months” buttons atop the graphics. COVID has dwindled down to a barely-there pathogen at this point.

The CDC still hosts hospitalization data. Not much is going on there. COVID in June 2023 is just not what it was two or three years ago.