Mailgun has made it progressively harder to get onto their flex plan, but it seems to still be there.
My recent attempt (July 2024) documented here - I initially thought it was gone, too!
Just checked my account, and I’m still on Flex:
(Of course, that doesn’t guarantee that you’re able to get on it as a new user, but the process that worked a month ago was ‘sign up for foundation free trial ($35/month), put in credit card, cancel plan before getting billed, accept flex when offered’)