A member of the former Commander rules committee dubbed them legal, but only for the person whose name is on the card - meaning that, as soon as they're sold, they go from functional playing pieces to pure collector's items
When it enters, it can exploit a creature, and it grants that ability to all your other legendary creatures, which is wild
Standard might struggle a bit more for this
Some examples: Swords to Plowshares, Teferis Protection, Chaos Warp, Great Train Heist (or other Spree cards, you pay extra costs but can go wild), Akromas Will, Deflecting Swat, Wear/Tear, and my personal favorite, Reprieve
One of Magic's best black enchantments and best black ETB cards, it offers strong value thats easy to understand and build around, and it even works well in generic decks that lack a reanimation or graveyard focus
Junk Winder Of course you simply must include Junk Winder in decks that drop a lot of cardboard like Lonis, Cryptozoologist and Eloise, Nephalia Sleuth