They will be reset. That was a bug. As much fun as people had trying to do a +21 King's Rest in pre-raid BFA gear, that was an oversight in terms of expansion transition. So no matter what your Keystone history is, that's being reset when Season 1 starts. You should go and do a Mythic 0 and will get a Mythic 2 Keystone for a random dungeon and can have fun pushing that up.
Mythic Keystone dungeons and ranked PvP are opening with the beginning of Season 1 tomorrow, and there are a few aspects of the first week of the Season to highlight.During this first week of Mythic Keystone dungeons, there is a cap on the increase of item level of end-of-run loot. That loot will stop increasing at Mythic Keystone +10 for this first week, which is item level 203. However, the Great Vault will be uncapped the following week, and loot from the Great Vault (on December 15 in this region) will reflect successful completions achieved during the week of December 8. The Great Vault will award item level 226 for completing Mythic Keystone +14, and other completion item levels can be viewed in the Mythic Keystone tab of the Dungeons and Raid window once the season begins.For this first week of the Season in PvP, item-level upgrades of Conquest equipment are capped at item level 220, requiring Rival (rank 4). Starting with the second week, (again that’s December 15 in this region), item level upgrades of Conquest equipment are uncapped. There are no caps on the Great Vault’s PvP considerations. Duelists (rank 5) or higher will receive item level 226 equipment from the Great Vault.Good luck out there!