In my experience (winter/spring 2020), it depended on where you were. In places that typically catered to Turkish people (grocery stores, gas stations, small shops, casual restaurants away from tourist attractions), prices were posted in Turkish Lira, of course, and that's what everyone used.
In places that primarily or exclusively catered to tourists, prices may have been posted in in Turkish Lira along with Euros, UK Pounds, Russian Roubles, and/or other currencies (US dollars were not at the top of the list).
Big ticket items (higher end hotels, balloon rides, expensive restaurants) might only have Euros listed.
A few places (resorts and restaurants in very touristy towns) whose clientele are pretty much all expats might only have prices posted UK Pounds.
Most businesses usually prefer if you pay in Euros (it's more stable than the Turkish Lira). Some ATMs do dispense Euros and other foreign currencies. I'd stick with a mix of Turkish Lira (for small things) and have some Euros for bigger ticket items. While most places readily accept credit cards, some will give discounts for cash (eg: balloon flights). Whenever using plastic to pay for anything, make sure you use a card with no foreign transaction fees, and watch out for sneaky Dynamic Currency Conversion.