Euros in Istanbul

No, take the Turkish Lira currency out of the ATM attached to a bank that is open for the best exchange rate.
Try to pay everything with a card or Apple or Google Pay where available.

Euros are useful only if a tour guide or hotel asks for them specifically. You are almost always better off paying in lira. Sometimes stores will display prices in euros at inflated prices, betting that you won't be willing to work out the exchange rates in your head.

Most of our expenses we charged to our credit cards in lira. We did pull lira from bank ATMs on 2 occasions but there was an extra fee attached we haven't experienced in other European countries.

Don't bring too much money in lira, it is not the most stable currency at the moment. So if you want a bit of spare cash "just in case", euros might be a good idea.

Euros are useful only if a tour guide or hotel asks for them
specifically. You are almost always better off paying in lira.
Sometimes stores will display prices in euros at inflated prices,
betting that you won't be willing to work out the exchange rates in
your head.

I was also asked to pay for a private tour in US Dollars (or lira equivalent). They knew I was American and about to head out on a RS tour, so I think they just did this assuming it was more convenient for me. I wouldn't get a lot of lira ahead of the trip, but having some when you arrive is helpful for tipping drivers. It costs a local person a fee to convert euros/dollars back to lira so they're going to charge you more for that convenience. Credit cards are widely accepted in bigger cities like Istanbul but many smaller vendors and restaurants will require the use of cash lira. Many hotels quote prices in Euros but my hotels charged me in the equivalent lira.

And FYI, in many places the ATMs are not attached to a bank - in many places there are big long rows of stand alone ATMs from multiple banks. This is handy because some charge higher fees (do research) and sometimes your card won't work so you can move to the next machine and try again. The value of the lira has plummeted and since the largest bank note is about $7ish dollars now, you'll likely need to visit the ATM more than once.

An older thread with tips for visiting Istanbul and Turkey is here, includes discussion of money matters and many other things: https://community.ricksteves.com/travel-forum/tours/so-you-re-going-on-the-best-of-turkey-or-best-of-istanbul-tour

Just returned from Istanbul yesterday. Had 2 separate private tour guides on different days. One requested payment in euros, the other in US dollars. I knew ahead of time about each, and brought the dollars. The euros I got at an ATM. I had to try several ATM's before the transaction could be completed, it seems that they don't all have euros. We used lira for tipping and small purchases, everything else we paid by credit card.

My last tour guide wanted dollars too.

We were in Istanbul last month. Every place took credit cards. The vendors at the Spice Market and Grand Bazaar took US dollars, lira, or credit. One restaurant only accepted lira or credit card (no USD). I wouldn’t buy euros.

My hotel in Istanbul offers a discount for cash over CC. We'll do the cash, and they do want it in USD.