It's only the Avatars—Basically, other than Goku and the angels the only people who can do it reliably is the protagonist of this one massively popular and regularly updating card battle game.
Though, Goku did gain the ability to access Ultra Instinct at will during the Moro Arc of the Super Manga, so... Yeah, I expect that it'll be replaced sometime soon.
Or he'll combine it with Super Saiyan or Kaioken.
I expect that this time next year we'll have a "mastered" Ultra Instinct version of the Heroes Avatars, so that's something to look forward to.
I'm expecting Glowing White members of Frieza's Race/Bio-androids and for the male Saiyans to have costume changes that involve taking their shirts off.