While the Turkish government isn’t quite as extreme as China when it comes to online censorship, you’ll probably want a decent VPN before your visit.
I’ve tried out a couple of different companies and have found ExpressVPN to be the most reliable.
Booking.com is currently banned in the country (at least when you search for domestic accommodation). However, there are actually quite a few Turkish hotels listed on there anyway. And many them don’t even appear on Hotels.com, which hasn’t been banned.
Over the course of my trip, I ended up making quite a few reservations with Booking.com and was really glad I had a VPN to do so.
Another major site that’s banned is PayPal. If you want to access your account at all during your travels, a VPN is a must.
While those are the only two major sites that I noticed were banned during my trip, Turkey has even gone as far as banning Wikipedia and Twitter in the past.