What exactly do you mean by "decentralized 2fa"? Just because something has the word "Google" in front of it, doesn't mean it isn't decentralized. You own your keys, and you are completely responsible for them. However I would argue that decentralized 2fa is unnecessary for the average person, as the chance of losing keys is much higher. Authy is tied to your phone number rather than your phone, making it much easier to restore lost data, without having to worry too much.
"My 2 cents" in regard of "decentralized 2fa":
Hydrogen 2FA uses cutting-edge cryptography and the blockchain to secure your accounts, transactions, and payments.
that said, relevant services should adopt
Hydro Raindrop so you could use that "decentralized 2fa alternative".