@leocg Received this from Fastmail Customer Support:
"Hello {me},
Thank you for contacting Fastmail support, I can absolutely understand your reluctance to move away from the operamail.com domain.
Unfortunately, this decision is out of our hands. Opera has decided that from June 2022, we will no longer be able to offer our customers email addresses at operamail.com
When Opera takes the domain Operamail back in June 2022, they will not be offering their own mail service with it, and they don't have any plans to offer email in the future.
So I am afraid there is no option to keep your operamail.com email address past June 2022.
This is why we are encouraging people to change their email addresses now so they have time to let everyone know about their new email address.
Please feel free to contact us again if you have any other questions or concerns. We're here to help.
Cheers,
{FastMail support person's name}"