Is cash required to prove the 20,000 baht financial requirement for the Visa 2 + 1 in Thailand?

There are two times you're likely to be asked for proof of funds. They are treated totally differently

You can be asked for proof of 20K baht or the equivalent when you apply for a tourist visa at some thai consulates <- in this case you show a BANK account, you CANNOT show cash.

You can be asked for proof of 20K baht or the equivalent when you stamp in to the country at passport control <- in this case you show CASH and you CANNOT show banked money.

I guess you like to apply for the Single Entry Tourist Visa which gives you 60 days of stay with the possibility to extend with another 30 days. You apply for this visa at a Thai Consulate and if they have the financial requirement you show a bank statement with the minimum amount required.