I think this would be awesome, as this would allow for people to automate withers without the worry that their farms might get reset.
This has been suggested in the past. Allowing Withers spawn isn't really possible in those dimensions. As every new dimension has it's own ID, we would have to whitelist each and every Spatial Dimension by hand. You can see this here Wither in storage cell dimension | MineYourMind Community
now that I think about it, people could literally make a super OP wither farm, bring it to the overworld with countless withers in tow, and then destroy the spawner, releasing them upon the world to wreak havoc everywhere they go. so, probably not the best idea... lol. maybe the last millenium? just some world where resets don't occur.