Mage boosting has been an issue in many aspects of World of Warcraft Classic over the past 3 years, causing degenerate gameplay and having an effect on the Classic economy.
After announcing the development of WoW Classic: Season of Mastery, Blizzard quickly followed up with comments regarding their plans to reduce the effectiveness of mage power leveling and get players to level their characters in the open world.
Blizzard also announced that they will be implementing the leveling nerfs that were introduced with The Burning Crusade, which makes it faster to level via quest completion.
With the 12-month cycle planned to release all 6 phases of content, this means players interested in the end game will want to level in a timely manner so these changes will fit in with the few changes that have been announced already.
Blizzard Are Looking To Nerf Mage Boosting In WoW Classic: Season of Mastery
Community Manager Kaivax said on the forums "we’re planning to take a look at mage boosting in the Beta for this release. We have some ideas for adjustments we can make to reduce its impact, especially in current hot-spots like Maraudon."
It sounds like there will be certain restrictions put in place throughout the popular dungeons instead of targeting the mage abilities themself.
There are currently no plans to increase sources of experience from other types of leveling methods, as it sounds like the team wants to bring more players back into the open world.