Warning: Contains SPOILERS for The Flash season 6, episode 10, "Marathon".
What "Crisis on Infinite Earths" means for Harrison Wells (Tom Cavanagh) is the Arrowverse's biggest (and saddest) change to The Flash, and it's one that can't be walked back. Limiting the Arrowverse to one Earth has permanently altered The Flash's approach to Tom Cavanagh's Harrison Wells characters.
"Crisis on Infinite Earths" lived up to the promise that it would be the Black Lightning -- onto one Earth. The first post-Crisis episode of The Flash explores some of the ways that the world has irreversibly changed.
In the midseason premiere of The Flash, Cisco (Carlos Valdez) expresses his frustration with Harry, who was killed alongside Jesse Quick as part of the Crisis. Thanks to "Crisis on Infinite Earths", The Flash will no longer be able to swap out its Harrison Wells character with a new version at the end of the season, so if Nash dies or decides not to stick around for season 7, Tom Cavanagh could be gone from the series for good.
This means that "Crisis on Infinite Earths" was the end of an era in more ways than was previously thought. Not only did "Crisis" end Oliver Queen's story in the Arrowverse, but it also put a stop to what has evolved into an annual tradition on The Flash.
The end of the rotating Harrison Wells tradition on The Flash was an unfortunate but also unavoidable change to the series. The existence of the multiverse in the Arrowverse is what made this tradition possible in the first place, and now the Arrowverse only has access to one Earth. And no show in the Arrowverse was more in tune with the concept of the multiverse than The Flash, which frequently dealt with visitors from other Earths, so it stands to reason that out of all them, The Flash would be affected the most. "Crisis" has certainly closed the door on one of The Flash's most important concepts, so it's worth wondering where the show will go from here, and what new concepts and ideas it will introduce in the episodes and seasons ahead.