Okay, as an informal singer, yet a connaseur of great voices, AKA Sinatra et al., there are a lot of things to take under consideration. I also have experience in health and aging too. I'll explain this shortly. I too, agree with many people that his voice is the best it has been since the early 90's. I'm listening to the Glendale night 2 concert right now, and the clear examples are Streets, and Bad, among others.
Bono over the years has tried to help his voice. Many times to little avail. Yes, it was reported that Red Hill Mining town didn't make the tour because he couldn't hit it. This was why the video was made and then quickly shelved.
Bono had stated that by the Zootv period he was smoking more. This progressed through the Popmart tour. And if you notice, listening Achtung through POP his voice was becoming more nasil, thinner and higher.
This happened for many reasons. What is ironic is that as weak as his voice was on the Elevation tour, he and the band, made a concerted effort for him to protect it as often as possible.
Smoking: He mentioned that started smoking more during zootv, and then it progressed. Then he made a concerted effort to stop ATYCL era. In part for his health, in part for his voice. He then mentioned listening to early albums, he singed "high, in a girly" way. So he started smoking again.
Protection: He has used humidifiers constantly during tours. There are pictures of him on the planes using them, to keep his lungs moist, and as Edge has said, to make him shut up. It has been known that every tour, he has blown out his voice at some point. Even, and especially during the "golden years" as some people like to put it. During Unforgettable Fire, Joshua Tree and LoveTown, as his voice was that beautiful and almost baritone, he completely shredded his voice. It was powerful, but as he put it, it wasn't controlled so he would shout and not sing.
But, in my opinion, other factors majorly affect his performance. In the 90's he started singing, as he put it. When you listen to the albums, he wasn't really shouting or belting the songs. Technincally this is also where he took a major turn. He started recording albums, from the couch, literally. ANYONE who knows singing will tell you this is the absolute worst way to sing. You cruch your diaprhagm and sing only from your throat, thus higher sounding voice, and more pressure on your throat. This has carried over live, where on the recent tours he has still sung very much from his throat. This will destroy a voice fast. I noticed that some times, on the Elevation tours, his voice would go in and out, and at some points, he would hit a huge resonating note, such as the beginning to Streets, and you could tell he was getting his whole diaphragm into it.
As one gets older, your nasal passages change a little bit. Your throat are become a little more calcified as do lungs. This happens without smoke, but smoke obviously doesn't help. Men's voices get higher. It's biological.
Stories about his sinuses are also true. He has said this is n concern to his wine allergies, he even passed out a few times from drinking too much red. But again, smoking doesn't help much either.
When I first heard songs from this album, I noticed Bono was using mroe open sounding vowels. A good choice when you want to sing loudly and open your mouth. So much of singing loudly, is actually about doing it safely. When I first heard ATYCLYB, I wondered why they put vocal choruses over the heaviest guitar power riffs, e.g. Beautiful Day an Elevation. One voice against 40 Marshall Stacks isn't the best.
But what I have noticed lately from the concerts is that when he sings in a regular register during this tour, his voice sounds slightly deeper. This could be from the Sinus surgery as some have stated. It also seems, that he is singing more from his diaphragm now, and less from his throat. This too is what makes a voice slightly deeper and more robust. His voice isn't as thin as it has been in recent years. If you listen to Streets, he seems to be singing it again, where over the last few tours, he was kind of shouting it. His high points don't sound perfect, but they haven't sounded bad.
All in all, I would simply say, he's easily sounding better. Now, I've written too much and I will go back to lurking.