I googled the phrase "percentage of transactions in US made with currency"
The first item returned was from the FED - which is probably an authoritative source - maybe.
The Federal Reserve Board of Governors in Washington DC.
www.federalreserve.gov
Money and Payments: The U.S. Dollar in the Age of Digital Transformation
This paper is the first step in a public discussion between the Federal Reserve and stakeholders about central bank digital currencies (CBDCs).
Long article - suffice it to say the article is about CBDCs.
"According to a 2020 survey, U.S. consumers used cash for 19 percent of total transactions (6 percent by value)"
I notice that they do not actually have "data" on transactions - which they should have - they used a survey.
"In some jurisdictions, however, digital payments have rapidly supplanted cash. While cash use in the United States fell from 40 percent of transactions in 2012 (12 percent by value) to 19 percent in 2020, other countries have seen more rapid declines.
23 In Sweden, for example, the proportion of cash payments fell from 33 percent to less than 10 percent over the same period.
24 In China, 50 percent of point-of-sale (POS) payments are made with a mobile wallet or app, while cash accounts for just 13 percent of POS payments."
Note that while 19% of "transactions" were cash, only 6% of "dollars spent" were cash.
We are so close to all digital currency that implementation only requires someone to develop a technique (because the tech is already existing) to capture cash based transactions for things like garage sales, tips, purchases of MD-20/20, Colt 45, and Menthol Kools, Pan Handlers that spit on your windshield, and donations to Political Candidates.