See this article about banning foreign currency use in Zimbabwe
Why Zimbabwe has banned foreign currencies https://www.bbc.co.uk/news/world-africa-48757080
That will be a big mess / joke, and it won't work!! We crossed into Zimbabwe on own on our recent B,SA,Z trip. Most stall and street vendors wanted only USD!! Also, there were vendors selling large denomination Zimbabwe bills as souvenirs. I didn't check the exchange rate at the time because I wasn't interested, but I suspect the "souvenir bills" were being sold at a fraction of the gov't rate- because the vendors wanted reliable cash (USD).
First, a little history for clarification. The original Zimbabwe dollar collapsed in 2008 after years of hyperinflation ($100 trillion notes were printed in 2008). This currency was demonetized and has no value as money and never will (only as souvenirs). After several years without a local currency, they introduced the Zimbabwe Bond Dollar in 2014 (first as coins, then as notes). These were supposed to be pegged to the US Dollar, but the value fell anyway. Last I heard they were officially worth about seven to the US dollar (officially, less on the street). The Bond Dollar is now being replaced by the new Zimbabwean Dollar (essentially the Bond Dollar with a new name). This is what the government is trying to mandate as the sole legal currency.
What does this mean for travelers? First of all, the lodging and meals are all paid by Tauck, so you can go to Zimbabwe and not spend any additional money. If you buy something at a hotel, you can put it on your credit card, but they will probably have to charge it in local currency with the usual conversion charges (if any) from your card issuer. Street markets will be problematic. They will want your dollars, but it will be a risk to use them. Buying local currency (if available) will also be a risk, since it converting excess back will likely result in a big loss (with inflation running 98% in Zimababwe, currency traders aren't going to want to hold Zimbabwe dollars). Your tour director and the local guides will undoubtedly give guidance on dealing with the currency issue. Bottom line, don't count on being able to shop the street markets in Zimbabwe.
I agree with Alan that it probably won't work. It would take some pretty draconian enforcement to make it work.