Is this a uniquely US issue, and if so, why? Other countries have enacted similar lockdowns, wouldn't a coin shortage show up elsewhere given similar circumstances?
In Europe it is generally cheaper as a merchant to take credit vs cash, because they have low credit processing fees. Because of this they take credit for pretty much any transaction. In the US, it's not uncommon to see places only take credit for purchases over $20, especially restaurants.
This must be semi-regional because it's definitely not the case in the Minneapolis area (or the Midwest in general as far as I've seen). Even mom and pop restaurants will gladly take your credit card. If it exists at all, the minimum I've seen is $5, I've never seen a $20 minimum (not saying it doesn't exist, but it's definitely not "common"). Of course, the US is a huge place, so it definitely could be a regional thing.