I just noticed this announcement at ProtonMail:

"Easily transfer your old emails to ProtonMail with the Import-Export app

"Our Import-Export app has exited beta and is available to all paid ProtonMail
users. 

"The app allows you to easily import your entire inbox history from
privacy-invasive email providers such as Gmail, Yahoo Mail, or Outlook.

"You can also export emails from your ProtonMail account to your hard drive for
an offline backup. The Import-Export app is available for Windows, macOS, and
Linux devices.
--- SBBSecho 3.11-Linux