My oldest source for that is Supernatural. I don't remember the exact episode, but in one of the early seasons the two brothers were in the car listening to music and Dean plays some Bon Jovi and Sam is like "Bon Jovi?! Seriously?!" And the reply goes something like "Bon Jovi rocks... On occasion"

Obviously not a serious source and obviously they don't say Bon Jovi isn't metal, just more of an amusing anecdote that I still remember almost 15 years later, for some reason.

But more seriously, Wikipedia lists genres there even if a band doesn't belong in a genre. Simply because they have some elements of that genre. And in Bon Jovi's case that was only on the early albums. You can go to the wiki page of their first 2 albums, for example and you'll see that the genr e listed are "glam metal / hard rock". Which means that even early on they weren't really metal. More like metal adjacent. Obviously beyond that they went more and more towards rock.

But I do agree that the definition of metal has changed a bit. But mostly when talking about the early days, not anytime recently. For example bowling for soup has a line in one of their songs that goes something like "when did motley crue become classic rock?!" So you might be onto something.

Beyond that... There's always gonna be silly purists that will try to gatekeep stuff. And metal has plenty of those. You can see it easily when there's talk about the more commercial or experimental bands like sleep token or whatever.