I can't find the flex plan in mailgun

on the site it just makes me do the test and then switch to the fundation plan of 35 $ / m but first I wanted to use the flex plan with the custom domain but I can’t find it, to know if it has been removed?

Did you reach out to Mailgun? They would be in the best position to help you with this.

Flex no longer exists. Sadly. One can no longer use Mailgun on any free capacity beyond the trial period.