I haven't seen firsthand proof that shounen manga are getting more violent, but let's say it's true. It does happen; popular media seem to follow weird cycles where the average level of violence rises steadily for a while, until you get to peak violence, and then there's a violence recession and things calm down for a while. In American film, the major peak was in the 1960s–1980s with movies like Night of the Living Dead and Cannibal Holocaust. In anime, peak violence was achieved in the 1970s and 1980s with titles like MD Geist, Violence Jack, Wicked City, and Urotsukidouji. It's entirely possible that anime is headed for another violence apex. It does seem like Western media might be heading that way too; the most popular Western works are violent stories like Game of Thrones and Breaking Bad.
These violence apexes have complex cultural and production causes at their roots. For instance, OVAs were an extremely common format for anime in the 1980s. Since OVAs are direct to video, they catch a lot less heat from censors than stuff that broadcasts on TV where anyone can see it, and it's probably no coincidence that many of the extremely violent anime of the 1980s were OVAs. In the West, cable TV and streaming have made it easier to get violent shows out there.
You could write whole books about the cultural side of this (and I'm sure someone has), but I lack the expertise to go into it too deeply. But there are a few social trends that seem to correlate with increasing violence in popular media:
- Widespread societal cynicism. This was true of America after 9/11; Saw came out in 2003, and Hostel came out in 2005. This was also true of America in the later years of the Cold War; Cannibal Holocaust came out in 1980.
- Widespread influence of militaristic groups. This was also true of America after 9/11, and in the 1970s and 1980s. In modern Japan, there has been a rising trend towards militarism; you can see the effect of this in anime like GATE, which glorifies the JSDF. The JSDF even tried to use moe as a recruiting tool in the same way that the US military uses first-person shooters.
- Fear of military, cultural, or economic invasion by enemies. True of America in the late Cold War and after 9/11, and also true of modern Japan, which fears the rise of China as an economic and military power.
So it does seem plausible to me that newer shounen manga is more violent than older material. But let's keep in mind that older shounen manga isn't necessarily as clean and wholesome as we remember. I haven't read The Promised Wonderland or Hunter x Hunter, but even the older stuff that I have read got pretty violent at times. Yu Yu Hakusho in particular had some pretty graphic sequences (and it's probably no coincidence that Hunter x Hunter is also violent, since they share an author). Rurouni Kenshin started out bloodless but got pretty graphic towards the middle of the Kyouto arc. Naruto and One Piece both have their moments. Even Dragon Ball Z featured Frieza getting cut straight in half during his final battle with Goku. I recall some fairly bloody scenes from what little I read of Saint Seiya. Even Yugioh, which definitely aims for the younger side of the 8-18 age group, had some scenes that were at least brutal if not actually gory. For that matter, shoujo has also historically been pretty violent—X and Ayashi no Ceres are hardly sugar, spice, and everything nice.
As to the last point, the audience for shounen manga is not uniformly young children. The most common audience is boys from eight to eighteen years old, though shounen manga is also often enjoyed by men, and women and girls as well. But even if we restrict ourselves to ages 8 to 18, it's not that shocking for boys ages 15 to 18 to be watching or reading extremely violent material. In the US, movies rated R are recommended for people 17 and older, but growing up, I didn't know a single person who waited until 17 to see an R-rated movie. I saw violent R-rated movies like Blade Runner and Aliens when I was 9 and 10. I've known kids who saw gore porn garbage like Saw and Hostel when they were 7 and 8 years old. I found that excessively young for such violent material, but I don't find anything particularly shocking about a fourteen-year-old potentially reading or watching Attack on Titan, at least as far as the violence is concerned. Also, keep in mind that Attack on Titan is a bit of an anomaly: Isayama pitched it to Shounen Jump, who passed on it because they felt it was too mature. It was snapped up by Shounen Magazine. Anecdotally, I've always found that Shounen Magazine manga of all genres have more sophisticated writing than Shounen Jump manga, so they probably lean to an older audience anyway.