Blizzard have now set a date for the end of Season of Mastery, and “on or after” February 14th, 2023 players will no longer be able to log in to the servers, but their characters will be available for transfer “for some time thereafter”. As noted in earlier blue posts, players will be able to transfer either to Classic Era or Wrath Classic realms.

SoM (Source)

As the Season of Mastery WoW Classic experience approaches its end, we will disable the “Enter World” button for all Season of Mastery realms on or after February 14, 2023. After that time, you will be unable to enter the game from the character selection screen.

We intend to keep your Season of Mastery character list available for some time thereafter, so that you can initiate free transfers from Season of Mastery realms to Classic Era or Wrath Classic realms. We do not yet know how long that access will continue to be available, but we will let you know once we do.

We suggest that all Season of Mastery players initiate free transfers for your characters as soon as possible.

Thank you very much!