I was doing my annual dust breaking fest and I scrolled thru my wild sets to see if I missed anything and I noticed we have a new tab of “Legacy” cards that (I assume) were the cards we owned before some or most became "core / unbreakable / uncraftable)
Are these safe to be broken for dust ?
Depends what you consider safe. You can dust them as long as you play standard only. I did it already.
If you disenchant a Legacy card, you’ll lose access to its Classic counterpart. You’ll no longer be able to use that card in either Wild or Classic modes.
Good, don’t play either mode
Sometimes the cards are also needed for tavernbrawls Maybe dust those, you never played with (even if they are considered good by other players) and keep your favorite ones. A lot of people regretted dusting everything later.
You can use them in duels too
Since I started paying for packs I don’t dust anything., with a few exceptions(Warlock and Warrior goldens).