This is the first time I got asked this question in my life: "Do Pilots Wear Diapers". At first, when I received this question, I thought it was a troll question, to be honest; but then I realised that people are genuinely curious about this.
So let's quickly answer this:
Do Pilots Wear Diapers?
Do Airline Pilots Wear Diapers?
People don't usually notice when airline pilots leave the cockpit to use the toilet. The reason is the curtains! When a pilot wants to use the lavatory, the cabin crew would block the aisle and use the curtains as well. That's what makes people wonder: "I was on an 8h flight and I didn't see the pilots use the toilet". You would actually be able to spot when pilots use the lavatory if you sit in the first rows but most people don't sit there, as it's usually business class.
So to answer the question. No! Airline Pilots don't wear adult diapers, they can use the toilet mid-flight if they need to.
I personally never met or heard of a pilot that uses adult diapers.
Do Fighter Pilots Wear Diapers?
As fighter jets are not equipped with toilets then the question: "Do Fighter Pilots Wear Diapers" would make perfect sense.
The answer will depend on the length and complexity of the mission. Generally speaking, flight training missions are an hour or so, and fighter pilots wouldn't need to take precautions here for any "Unexpected Biological" situation.
However, for combat missions which are long and complex, fighter pilots might opt to use adult diapers but most would use what's called "Piddle-Packs". Put simply, it's a urine bag. The Piddle-Packs come with either compressed sponges or hydrophilic granulated powder.
But it's not that easy to use, as you have to use it quite a few times to get the hang of it. Don't forget! You would be peeing while flying the aircraft and manoeuvring. It's more difficult than it sounds.
Do Private Pilots Wear Diapers?
Small aircraft also come with no toilets so: "Do Private Pilots Wear Diapers?". I never did during my training, as I always time my toilet needs just before departure, this way, I would be comfortable during my flight until the end as usually small general aircraft fly for 3 - 4 hours max. As long as you pre-plan, you should be fine!
Few pilots use the Piddle-Packs just as fighter pilots do and a disgusting truth is that some pilots use an empty bottle to pee in if they are travelling solo and have to pee.