Blizzard have reminded players that they only have until July 28th to transfer their characters from Season of Mastery realms, as the servers will be completely unavailable then. 

Season of Mastery (Source)

Season of Mastery realms have remained available for login since we first closed off gameplay months ago.

They will be removed from the realm list in-game on Friday, July 28.

If you still have a character that you wish to move for free from a Season of Mastery realm, please do so immediately.

Season of Mastery realms will become permanently unavailable on July 28.

Thank you!