No, it wasn't always nebulous. Roguelike was a well-established genre for decades before it got hijacked and now means nothing.
Like all genres, games within the roguelike genre (or what some people call "traditional roguelikes") have some variance. But if you played two games in the "traditional roguelike" genre, you'd definitely feel the similarities.
These days if you pick two random games on Steam with the "roguelike" tag, you're going to get two experiences which are not even reminiscent of the other.
The meaning degraded much earlier than just a couple years ago. People thought it was cool so they latched onto it. It seems like that process started 7-8 years ago, maybe even a bit further back.
I played a *lot* of rogue in the early 80s. I can't remember a single game marketed as a "roguelike" that I've played that reminded me of playing rogue.
I haven't played Rogue, but I've played a lot of Moria, Nethack, and AdoM. Those are what I think of when I hear "Roguelike", although even AdoM might be stretching it a bit with its massive non-random outdoor area.
This is great. Old school game dev where youd built the whole engine optimized for the game rather than using an over generalized mess like unity or unreal