I thought it was "line" as in "crossing the Bishōnen Line", personally. I can't say anything else about it, but I could see it being unclear and leading to misuse.
Read Otr of the Flame (She/Her)It's named for the Adventurers! quote at the top of the page, FWIW. That's probably why the name is so odd - when it was named, Adventurers was hugely popular and many readers were familiar with it (which shows the dangers of trope namers, perhaps.)
I think the name is generally fine, definitely comes from when anime fandom terms were dominating the site but the idea is pretty straightforward.
That said, I think they took the inspiration and made a definition that is not quite in line with a lot of examples. It's heavily based on the Frieza method where a sequence of transformations become more monstrous until a sudden switch to a more gentle, attractive form. But the page image is of Cell, where his original form is the most alien, his second form is more muscular but more humanoid and then his final form is almost human. I think it's still a fine example of the broad idea, but the majority of non-shonen examples just has a single "monstrous form to pleasant form" and doesn't feature the ramp-up and drop off the definition is implying. This would be resolved with a minor definition tweak.
Comics are just words and pictures. You can do anything with words and pictures.One thing to note is that, historically, back when the site had looser rules, most of the tropes with anime fanspeak in their names were supposed to only be used with anime/manga. I'm pretty sure we don't like keeping to this rule anymore, I believe tropes like Yandere and Chuunibyou were once under this restriction, but now are free to have examples from anywhere, regardless of cultural origin. You would need a wick check to see, but it's possible Bishōnen Line might be underrepresented in media outside anime/manga.
I think you have your timeline a little mixed up. Eddie tried to make those names anime only (in this thread
) to counteract the crazy inscrutable names that the anime fandom (at the time extremely militant and resistant to change) kept pushing, by giving the non-anime fans actually understandable names, while letting the anime fans have their own trope names for tropes. I don't think it went anywhere and in the end we decided to focus on Clear, Concise, Witty for trope names (though bad ones do slip through) instead of naming them in Japanese
I don't think anything in particular came from that thread. Long before, a good portion of tropers had this notion that you could neatly separate tropes by medium. This mindset resulted in Japanese Delinquents being "Delinquents", Lyrics/Video Mismatch being "Narrative Non Sequitur", and "Saving The World" being videogame-only. Of course this was undermined by plenty of other editors who had no hesitation in putting in entries of other media. This notion of separate tropes was never in practice, but it was a strong idea at the time.
I took a look at Related, and it's dominated by anime and videogames. Much of it, I expect, from the influence of Dragon Ball.
Stories don't tell us monsters exist; we knew that already. They show us that monsters can be trademarked and milked for years.Actually, as mentioned above, the Trope Namer is a Western webcomic so I don't think "anime fanspeak" came into it as much as with other Japanese trope names. That said, the only examples in the oldest copy on the Internet Archive
all come from Dragon Ball.

Bishōnen Line, quoting the laconic page, is "the point at which an evil creature transforms into a more humanoid form instead of getting increasingly monstrous." Nothing about the trope name denotes that it's about villains specifically and it only vaguely talks about transformation ("Bishonen" implies turning into a more humanlike form and "Line" as in evolutionary line, but I'm not too sure about that one).
I feel like it also gets misused sometimes (applied to neutrally-aligned/heroic characters sometimes) because of its name but since I haven't actually checked I don't know
Guys how cool would it be if Super Megaforce had a green ranger...