disclaimer: Those are NOT facts, those are my OPINIONS and impressions based in what I used to see and read on internet. So it's a 100% biased thread
My intention is not to hate or shade any group, please I just want to share my impressions being on kpop for so long and show the differences in the treatment of groups I have been following for a long time
The thread is a bit long. If you want to see my opinion about another group please comment
So let's start 
How it was: BTS was far and away the most hated group. The amount of animosity they used to get was something I had never seen for any group with their level of popularity. Generally speaking, most of the group hate starts when the group is either approaching the top or at the top, but BTS seemed to evoke an amount of hate even when they were significantly less popular than EXO.
How it is: I'd say the attitude towards them has warmed a lot with time, even if they are still very far from being a "loved" group by 3rd gen stans. They seem to be quite liked (or at least respected) by 4th gen stans. They still have their haters, but overall, I no longer see a collective hate train against them to the same degree they used to have. If anything, I see more hate towards them coming from non-Kpop fans as they sometimes clash with Western singers on charts and awards.
How it was: Blackpink was the token group for 9 out of 10 new non-Army Kpop fans in 2016 and 2017. Many people called them a weaker version of 2NE1, as well as a long list of posts dooming and glooming their future because of YG. But overall, they were a liked group.
How it is: Blackpink is now a hated group, or at least a group Kpop fans (mostly 3rd gen stans) actively dislike. I believe they are a group the Kpop community gets tired of after some time and are now liked basically only by their fans (inside the Kpop bubble) and some 4th gen stans who discovered Kpop after 3rd gen was already over. The saving grace for Blackpink is that they don't need the Kpop community. They are pretty well-liked by casual music listeners, especially in East and Southeast Asia, where their status is god-like.
Twice
How it was: Twice was a DESPISED group. I'm using the word "despised" instead of "hated" because I feel it better describes the attitudes iFans had towards them. I believe Twice's popularity made iFans realize Koreans liked cute concepts, and it didn't sit well with iFans to see what they considered an inferior version of Girls' Generation smashing Korean charts with songs like "Cheer Up" and "TT". Twice suffered systematic hate campaigns. I remember people mass-disliking their YouTube videos and trending edited MR removed videos to expose how bad singers they were. I still think to this day that Twice suffers from the insane amount of hate directed towards them.
How it is: Considering how disliked the group was, they fared pretty well with time, to be honest. Twice is a really well-liked group now on iFans' side, with many casual listeners. Overall, they are one of the most liked 3rd gen GG. I'd say they are like Girls' Generation and have aged like wine with time.
EXO
How it was: To this date, I still find EXO the most liked "top group" in Kpop (debatable since NewJeans' debut). They seemed to have many haters on the K-side, but not so many on the i-side. The fandoms that actively hated EXO were, for me, pretty clear: VIPs and Armys. The rest either liked or simply didn't care for them.
How it is: 4th gen stans don't give a damn about EXO. Their catalog and recognition mostly come from 3rd gen stans who are still active. Overall, I'd say their songs have aged well, being more pop and R&B-ish and still receiving praise, especially for not resembling current BG music. And of course, their vocals are always subject to praise.
How it was: Those who liked RV always stated they were the best thing in Kpop at the time. Overall, I'd say they were a well-liked group, with most of their hate being one-sided. Their music could be controversial, especially because iFans were never that fond of their Red side, preferring their Velvet side.
How it is: They are still mostly regarded as a group with a very good discography and talented singers. They lack the same level of hype they used to have, but overall, it's hard to deny they have a very respected and well-liked discography.
How it was: NCT was a victim of strong bullying from all sides. They used to be called flops, and I believe till this day NCT is the precursor and the true synonym of "Boygroups only release noisy shit". Nevertheless, people used to dislike NCT as a concept of a group too, being separated into multiple units. This concept was always subject to scrutiny with Kpop fans, as the concept was too confusing and there were too many members.
How it is: To be honest, nothing has really changed. NCT is still very ignored, and people only bother to talk about them when it's time to criticize their songs. People stopped calling them flops, but they are still not liked, even by 4th gen stans.
How it was: Mamamoo was a very ignored group on i-fans' side. There isn't much to say because they were very far behind. Back in the day, it seemed like there were only three girl groups i-fans cared about, and the others were all boy groups (the opposite of today). Nevertheless, just like Red Velvet, people who were fond of Mamamoo were very vocal about their talent and music quality.
How it is: Having found significant success overseas only very late in their careers, Mamamoo is a group that most people feel ended "too soon". It's similar to what would happen if (G)I-DLE disbanded tomorrow after "Tomboy", "Nxde", and "Queencard". Overall, they are respected but less remembered.
How it was: Just like Mamamoo, Seventeen was a group that only their core audience cared about, but they were overall very well-liked. They used to trend regularly and receive praise for their strong and detailed choreography and perfectly in-sync dancing. They seemed like a very promising group that was going to hit it big, or at least that's what everybody thought at the time.
How it is: Their core audience has increased, and that's about it. They don't even seem like a top group anymore. I'd say they are neither hated nor liked; they are just there. It's a strange situation where they have definitely grown and never flopped, but even with all the promise, they failed to have a mainstream breakthrough moment. The attention seems to have shifted towards newer 4th gen groups (TxT, Stray Kids, Enhypen), which diverted some of the audience that otherwise might have gone to Seventeen.
