In a nutshell, the developer was developing a Roguelike, but because it did not pan out the way he intended to, he got inspired by games he found on Steam to make another quote-on-quote "Roguelike", wrongfully thinking he was still developing a Roguelike and that misconception came from a spread-out misinformation that still gets perpetuated to this day.
You are saying that you expected an authentic traditional roguelike experience from a developer who, by your own account, doesn't even know what a roguelike is.
What seems to have happened here is that the developer did not like what they were making, so they made something else instead. You seem to be suggesting that the reason this happened is that language is insufficiently prescriptivist, and not that the developer just saw something they liked better and changed their mind.
Secondly, this asinine problem makes it harder for me (and other people like me) to find games that I like, thankfully the Dungeon-Crawler tag still works and that is the only way I can still find Roguelikes on gaming sites such as Steam, Itch and Gamejolt.
I don't see why that would be the case. Not all dungeon crawlers are roguelikes, not even close; and roguelites are probably almost as apt to be labeled dungeon crawlers as roguelikes are, given that dungeon crawling is typically considered a theme, not a genre.
I decided to try this out for myself. Both The Binding of Isaac and Slay the Spire, which you keep bringing up as undesirable, are tagged "dungeon crawler" on Steam; in fact, when I browse the dungeon crawler tag on Steam, both of those games are prominently displayed in the banner at the top of the page. Glancing over the actual recommendations, I see a handful of traditional roguelikes, as is expected, but I mostly see other genres. If I were actually looking for roguelikes in this list, I would probably be disappointed. The roguelike list is, as you say, a mix of roguelikes and roguelites, but it looks more promising than the dungeon crawlers.
Filtering my own Steam library for dungeon crawlers, I see JRPGs, action RPGs, first-person shooters, arcade games, puzzle games, board games, a platformer, and more. Few of these are what I would actually want to see when searching for dungeon crawlers.
If you still find the dungeon crawler tag a reasonable and effective filter for roguelikes in spite of all that, then why do you find the roguelike tag so unusable? I sympathize with your difficulty in finding what you want, but most highly specific sub-subgenres do not have a commonly agreed-upon label at all, let alone one as precisely defined (if frequently misapplied) as "roguelike." All anyone has to meet their specific tastes is an approximation, which then has to be sifted by hand. It isn't just roguelikes.
By the way, I know of folks who would take umbrage at grouping traditional roguelikes in with dungeon crawlers. To them, dungeon crawler is a term with a very specific, long-established meaning (which does not include Rogue) that has been diluted into uselessness, making it difficult to find the true dungeon crawlers amongst all the mislabeled pretenders.
Nevertheless, while language is subjective, it needs to serve its purpose. I can not imagine how a conversation between a fan of Rogue and a fan of Isaac would go like when the term that even link these 2 games together have different definitions, the other of which is wrong
Why can you not imagine it? I would think that they would discuss the games themselves, rather than being confused and deadlocked over a label. I can't remember ever having that problem.