If I recall correctly, Espeon and Umbreon were also usable in
Fire Red and Leaf Green. It's just that without the day/night mechanic, the means to evolve Eevee into Espeon and Umbreon were unavailable. However, they could still be traded in. (This was not possible until
Pokémon Colosseum, however.) Not only were they available, they had unique Pokédex entries, and the same goes for Leafeon and Glaceon in
Heart Gold and Soul Silver.
I think a better example in
Fire Red and Leaf Green is Crobat. You are restricted from evolving your Golbat into Crobat despite there being a happiness value--until the postgame, upon which if it's happy enough, it will evolve the next time it levels up. The same thing happens with any other Pokémon that evolves via happiness, like Chansey into Blissey, as well as Baby Pokémon as a nursery becomes available in the posrtgame Sevii Islands and gives you full access to all breeding mechanics at the time.
Meanwhile, Magmortar is available in
Heart Gold and Soul Silver, but the Magmarizer--the item needed to evolve Magmar--is found only on Cinnabar, preventing you from using it until the postgame. Similarly, the Electirizer used to evolve Electabuzz into Electivire is found in Cerulean Cave (one of the last areas in the game), the Dubious Disc used to evolve Porygon2 into Porygon-Z is in an area requiring Rock Climb (whose functionality comes from defeating Blue in the postgame), the Razor Fang used to evolve Gligar into Gliscor and Razor Claw used to evolve Sneasel into Weavile is obtained at the Battle Frontier, etc. The Ancient Power ones were, to my knowledge, restricted in much the same way as happiness in
Fire Red and Leaf Green.
With this pattern in mind, I think that in the case of Sylveon, you will be allowed to let Eevee learn Fairy-type moves before the end credits roll but will not be able to evolve it into Sylveon until afterwards, once the blocks get lifted.
Honestly I'm starting to really warm up to the idea that in post game we are able to dex radar post gen 4 pokemon like how you could dexnav gen 4 and 5 pokemon that obviously never appeared in the ORAS dex. I really want to use Toucannon and Eelektross again. Especially my best eel boy
I really want to have Minior be on a team again. Minior's so fun. It's cute and kind of reminds me of a Kinder Surprise Egg.