To say Bakugo and Deku canonically hate each other is a very wild thing to claim. I understand just not liking them as a pairing but Horikoshi has shown that the two of them do care about each other in the series
The importance of a mainstream Shonen having gay relationships, is it treats it as normal.
Yes, there are mangas/anime that are categorized as yaoi or yuri. But the thing about that media is that it's only written for gay people (if it's not written as a fetish.) Those shows are good, but the queer relationships are established from the beginning, as does the yaoi/yuri label.
For a shonen manga like My Hero Academia to have a gay relationship, it completely changes how gay people are represented in anime. Mha's intended audience is a variety of people, rather than just queer people. It had a huge number of het boys and girls watching it because the main focus is heroism rather than relationships. THAT'S what makes Bakudeku so important.
For bkdk to be made canon, that means a relationship was developed and added as a side plot rather than having full focus shifted on it. And it would be a queer relationship, on a Shonen. That's revolutionary, because it puts queer relationships on the same casual level as straight relationships. Think of aot, and how much of an impact Ymir and Historia being queer had, as a casual queer relationship between two important characters. Now imagine that as the MAIN characters. Am I making sense? Are you understanding what I'm trying to say?