is there any implementation that uses hardened keys?
Well, Bitcoin Core uses m/0'/0'/i' as default, so hardened keys all the way down to the address index.
I suppose some other older wallets which still use BIP 32 for their derivation paths rather than the more common BIP 44/49/84 may still use hardened keys at all levels, but given almost all wallets now follow BIP 44/49/84, I'm not familiar with any.