Can I still apply for a Covid extension to stay in Thailand after May 20th?

your friend doesn't know what they're talking about The current covid extension program runs until May 24th.

NOW whether you'll get another covid extension with a 2+ year stay history on amensty/covid extensions is another story entirely.

All you can do is go to your immigration office and apply. They'll either let you get one or they'll tell you to leave the country OR they could sell you a 7 day extension denied stamp that gives you a week to get out (for 1900baht)