Many of the wallets that supported sign in with lightning are not wallets that you can attach as sending wallets.

It creates a situation where you use a wallet like Breez wallet or CashApp to sign in, but then you can't use it to send zaps, so you have to attach another wallet.

We had enough people get confused by this that we figured it was more straightforward to remove the option for new signups. If you were using sign in with lightning, you still can.