Some exceptions: credit card details, city, state, and ZIP fields. I’m not mentioning the Name field because it should be a single Full Name field, instead of First Name and Last Name.
To quote Baymard (source):
Forms with multi-column designs often lead users to:
- omit fields, simply because they do not notice them or because they misinterpret the meaning of the multiple columns, or
- fill out unrelated or incorrect fields because they misinterpret them to be part of the needed input.
And actually, Shopify implemented this, their checkout form is a single column (except for the name fields and the country / state / zip.
Their second column has a summary, discount code, subtotal, shipping, and total… and a different background color.