Blizzard clarified that Mythic+ rewards for Season 4 will still be obtainable after the season ends on October 25.
The rewards include Restoration Deathwalker and dungeon teleports.
Mythic+ Achievements and Rewards
RESTORATION DEATHWALKER MOUNT
With the death of Zovaal the Jailer and the ascension of a new Arbiter, the Shadowlands is returning to its Great Purpose. Some deathwalkers recognize the role that mortal heroes have played and are willing to serve as mounts. Players wishing to earn the Restoration Deathwalker mount must reach a 2,000 M+ rating.
LAST STOP, ALL PASSENGERS MUST DEPART
Completing the Mythic Keystone level 20 or higher within the time limit, thus awarding portals to Grimrail Depot, Iron Docks, Operation: Mechagon, Return to Karazhan, and Tazavesh will be going away when Dragonflight launches.
For an extensive list of things going away in the pre-patch / Dragonflight, check out our latest guide!