Update as of Gen 8
Thankfully, they changed that limitation in Sword and Shield. Now, using a Rare Candy on a level 100 Pokemon (with the right requirements met), it is possible to evolve level 100 Pokemon. They won't gain a level from the Rare Candy, however.
So if trading your Eevee to Gen 8 is an option, you can evolve it into Glaceon that way :) Plus, they changed the Evolution method to the Ice Stone, so likely it'll remain this way Gen 8+