unveiled mods in 3.6

If we unlocked them in 3.5 do we need to do it again in 3.6?

That would be a real bummer.

Most likely yes. They'll be unlocked in standard after league ends.
Before the master changes, we had to lvl up the old masters every league to get the crafting recipes. Don't see how this will be any different.

It is very likely veilded enchants will have to be re-acquired in any new challenge league. This is crazy as some of the rare mods are way too rare. Betrayal is a league where I spent most time in end-game, leveled character to 95, another to 98, run hundreds upon hundreds of red maps and I have not found many powerful enchants, e.g. +1 summoned totems. Picking up and unveiling mods is also one of more annoying aspects of Betryal so I'm not looking forward doing it all over again in Synthesis but this time with more crap to pick related to Synthesis.

Syndicate will be way rarer though. And we'll have to re-pick all the story-related ones, delve ones... Crazy.