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.
that makes sense.
i believe wallets that are designed like Electrum that have focused on both BIP-32 (HD wallets) and cold storage mode (offline-online) must use non-hardened addresses because if they don't the online-offline mode won't work since it requires master public key only on the online wallet.