I don't think new evolutions of past event Pokemon fits this topic (in general).
Example, Fly and Surf Pikachu in Gen 6 will evolve to normal Raichu, but if you keep it, up it to Gen 7, you'll have Fly and Surf Alolan Raichu.
It's something obtainable, if you thought about it. (much like evolving DW event Eevee into Sylveon)
What @Got_Eevees had in mind, was the eeveelutions itself (Vaporeon, Jolteon etc) was released as DW event, so why not "make one" for Sylveon.
Well, my answer to that is: it's not unreleased in terms of:
1. It was drafted outside of the game but never released (test wonder cards, proto cards, other external testing software or distribution devices)
2. It was programmed into the game but never released (debugging tools, leftover internal event data, programmed event went unused)
Something like Flash Fire Entei being unreleased in Gen V and VI fits the bill better (exists in coding HA never released),
as compared to DW Sylveon in Gen V, that didn't even exist then (does not exist as data or coding, cause the idea of Sylveon probably wasn't even conceptualized at that point)