Core Lightning users were using c-lightning rest, a rest server made by a third party, same fev as RTL, to connect to all wallets. But this will be deprecated now that Core Lightning directly integrate a connector system called rune, which is much more powerful, allow to set limits and advances rules to increase secruity. RTL was the first to be compatible, following by BTC Pay Server I guess and Zeus will implement this soon as well.

I'm not using BTCPay with the provided lightning instance, so I'm not concerned but I assume you can still add c-lightning rest in the meantime, or just wait for Zeus to implement it, no date thus. 

 Ha ha, I didn't even think to add it back as a custom snippet! Thanks dude.