If you're into anime (Japanese animation) or light novels (Japanese young adult novels), you may have noticed that in recent years, titles have gotten a little... long. And not just long, but distinctly sentence-like. You'll have titles like "So I'm a Spider, So What?", "I've Been Killing Slimes for 300 Years and Maxed Out My Level" and "Is It Wrong to Try to Pick Up Girls in a Dungeon?" Compare this to previous titles like "Bakemonogatari" (a pun on 'bakemono', monster, and 'monogatari', stories).
YouTuber RedBard has already made a video covering when this change happened, which I recommend watching:
This looks at the change of length in English language translations. I also recommend this accompanying piece at Otaquest, which looks at the change in Japanese, and this genre-based analysis by jgeekstudies.
Both Red Bard and jgeekstudies seem to agree that light novel title lengths began increasing dramatically in 2014. The jgeekstudies paper mentions that the long titles are most common in the fantasy and isekai genres. 'Isekai' is a genre based around a person who is transported to another world and has to live there now. "Sword Art Online" is an example of a popular isekai work. The three titles I mentioned in the open paragraph are also either fantasy or isekai.
Red Bard speculates that light novels have become so long as a way to give a very neat plot summary and hook readers in right away. Light novels are displayed with the spine outwards, so having an attention-grabbing title can cause a potential reader to check out the book and its actual summary in a way a one-word title might not.
I'm not a big consumer of isekai anime, so I don't have a representative sample, but I have watched "I've Been Killing Slimes for 300 Years and Maxed Out My Level" and I don't know if I'd really call the title a good summary of the story. The plot begins with a young woman who dies of overwork, and is reincarnated as an immortal into a video game fantasy world by a goddess who takes pity on her. She decides to live a carefree, slow life and only kills a handful of slimes each day. After 300 years, she has, indeed, 'maxed out her level' by killing slimes.
However, this barely has any relation to the real story, which involves her becoming an adoptive mom for two slime spirits, building a chosen family, and generally getting into hijinks. Her having maxed out her level is more an excuse for her to be overpowered; her killing slimes for 300 years is related to the overall theme of avoiding overwork and choosing a slow life (raising her level slowly instead of grinding out powerful enemies every day). I would consider the title more of a hook of the absurb elements of the show than something that really has much to do with the rest of the plot.
Indeed, I think it's worth pointing out that these sentences are all humorous, or at least a little absurd. "So I'm a Spider, So what?" could have been called "I got reincarnated as a spider" - already a silly premise - but goes the extra length of being defensive about being reincarnated as a title. "Is it wrong to try to pick up girls in a dungeon" takes a funny concept and also makes the asker sound defensive or curious.
It's funny to consider that light novel titles are becoming full sentences compared to English language pop songs, which have gone the opposite direction from favoring sentences or titles to single words or phrases (previous posts here and here). I mention in a previous article that having a song title be a sentence picked out from a song's chorus was a good way to make your song memorable, which was important in a time before Shazam and internet on cell phones to look up song lyrics. Today, it's relatively easy to find out what a song's title is, so there's little risk to naming it something short and sweet from the beginning or end of the chorus.
It's just another part of enregistration, where something becomes popular due to some reason (an early innovator who struck gold with a highly recognizable example, random chance causing a cluster), is copied by people, and ultimately becomes a genre marker. If I see an anime titled something like "Yeah, it's true - my little sister got reincarnated as a yogurt after beating a video game", there's a pretty good chance it's going to be some kind of isekai or fantasy adapted from a light novel. The fact that it's so easily parody-able, as I just showed, also makes it very easy to produce - find something absurd in the premise of your book, add some kind of emotional coloring to it with a discourse marker, and you have a title that can intrigue readers and make them laugh while not challenging them stylistically. Profit!
Will light novel titles continue being sentences? Probably for a while, until people get tired of it and are ready for something new. Titles of things tend to fluctuate over time, and trends come and go, just like one-word titles for songs. Nevertheless, it's fascinating to see just how fast the convention of "light novel titles as sentences" got adopted and enregistered.