^ What the post above me said, exactly.

Here's some clarification for some of the unclear parts:

For synthesis: +1/+2/+3 items are now all treated like the old +3s used to be, meaning they give the exact same starting quality and durability (300ish and 150ish, respectively). Only NQ and +1 items can be created now. The dying process is instant and unfailable, and your gear will retain any materia attached to it.

For gathering: Only NQ and +1 items are obtainable now.

For preexisting gear: +1/+2/+3 items now all have the same stats and have been modified to have the stats of what used to be +2 items.

For preexisting materials: None of your old items have had their quality change, so any old +2/+3s will still be there, but they are effectively identical to the new +1... you just can't stack them together.

Why they didn't just crawl their database for +2 and +3 items and change them to +1 items, I'll never know.

I think there's still the synthesis HQ overhaul coming in 1.21.