What documentation is required to prove remote income for a DTV visa in Thailand?

work contract + certificate of employment stating remote work.

but these days they want to see 3 months of bank statements. so these would show income.

just upload as much quality information as you can.

If you are employed you would show your contract, a letter from your employer giving you permission to explicitly work remotely from Thailand and stating you won't work with any Thai entities or clients inside Thailand, payslips and bank statements highlighting the pay entering your account. If you are self employed you'll still need the letter but you can write it to yourself, a portfolio of your work, invoices and corresponding receipts highlighted in your bank statements, tax registration, etc

To show you had remote income you would show your bank statements highlighting the income deposits.