List of Custom fields that can be used in a landing pages form:

• Currency
• Date
• Date/Time
• Drop-down
• Email
• List Box
• Name
• Phone Number
• Radio
• Text
• Text Area
• Website
• Yes/No

List of Custom fields that cannot be used in a landing pages form:

• Day of Week
• Decimal Number
• Drill-down
• Month
• Percent
• Social Security Number
• State
• User
• Whole Number
• Year

This article is talking about the allowed form fields available to use. This is not discussing merge fields in a landing page. No merge fields are supported on the Landing page itself.