I know exactly which checkbox you mean, a little blue slider thing to indicate I want to continue without an account. It used to be there.
I guess I'm the unwilling guinea pig in some sorta A/B test that Paypal is doing.
EDIT: Solved it. I have to open a private mode window, log into itch again, make the purchase and then tell paypal an e-mail address I've never used with them before. The option to skip account creation now appears again.
The second time I use that e-mail address again... the option is gone. So I can do at most one transaction per e-mail address until I'm forced to register. All this with the same address and credit card data.