they didn't not-promote it... not sure what that would've even entailed... back then you made a video and you didn't promote or not promote the video (like on social media), rather, the video itself served as promotion/advertising for the album, you sent it to MTV and if you were a big artist and the video was judged as likely to be popular, they played it a lot, if you were a semi-big artist and the video was judged as likely to be popular with some of MTV's viewers, they played it somewhat, during certain shows at certain times of day (devoted to that artist's genre) and if you were an artist with no mainstream popularity and the video wasn't judged likely to be popular it wouldn't be played at all.

radiohead videos, such as knives out, got played a medium amount simply for existing and being radiohead. i can confirm knives out was indeed played on MTV because i remember being at a relative's and they were watching MTV and it was on, in between other new/reasonably popular artists. there was no need for the band to talk about the video or promote it. it was perfectly normal not to. the video itself was the promotion. it promoted sales of the amnesiac cd. interviews were another way of promoting the cd. but they rarely talked about their videos in their interviews. after all, most of the video concepts (not just this one) do not originate with the band. if you want to hear about what karma police video means, ask jonathan glazer not radiohead. radiohead never direct their own videos (until daydreaming actually, which they codirected). so thom actually said nothing at all about the knives out video in interviews... but hey, he also said little or nothing about the other videos they were doing, so it didn't seem like he was dissing gondry. it was a perfectly normal radiohead video release- as of 2001, it got played much more than any of their videos since karma police actually (there there would probably have even more mtv success two years later, although by that time, i already watched videos on the pre-youtube internet rather than trying to see them on mtv).

the controversy with knives out originates several years later, when gondry was interviewed about it around the time of his movie eternal sunshine of the spotless mind. you have to recall that although gondry was a genius of the art of video and arguably the best filmmaker of the '90s, his actual name was little known to the general public, up until the 2000s, which is when he began making feature films as well. i remember when it was announced that gondry would direct a video for knives out, most radiohead fans on the fansites at the time were actually disappointed because spike jonze had been rumored to direct knives out, and when it turned out to be this random french dude, very few people were even aware he was the same random french dude behind hyperballad, isobel, bachelorette, human behaviour, around the world, sugar water, everlong, and protection. almost nobody had heard of michel gondry (because he hadn't made any feature films yet until that same summer, and that film- human nature- was wrongly trashed by critics) and everyone had heard of spike jonze (because he'd already made a feature film, being john malkovich, which got lots of awards attention two years earlier). in retrospect, as awesome as spike jonze is with his back catalog and then a masterpiece like her, still, pretty much most cinephiles or music video aficionados would recognize that gondry is more of an original and the greater genius. at the time however, even radiohead members may not have sufficiently respected gondry, seeing him as a budget-price jonze (it's possible jonze even agreed to do the video originally, then wasn't able to, and gave the assignment to his friend). whatever the reasons for radiohead not being happy with the video however, the video was promoted perfectly normally at the time, and that information only came out two or three years later in an interview with gondry, where he revealed that radiohead didn't even let him put the video on his own 2003 dvd collection (which is, assuming it's true, complete bullshit, since gondry not radiohead was the artist behind the video and radiohead had no post-okc video collection of their own and no intention of releasing one). that's when gondry realized just how much thom apparently hated the video. and gondry's pov at the time was also much more sympathetic to most people than radiohead's, since in between doing the knives out video and being interviewed about it, he had directed massively popular and award winning videos for kylie and the white stripes, and by 2004 he had belatedly become famous as director of the year's most critically acclaimed film, eternal sunshine, and the themes and imagery of that film are actually very very close to his knives out video (making it seem super important in retrospect- probably why he was so angry the band didn't even let him put it out on dvd!).

since that time, several years after the video was initially released like any other video, it became more "rare" due to the dispute between gondry and radiohead (or rather, radiohead's disrespect of gondry). it may be that radiohead asked EMI or lately XL to remove the video from youtube, i don't know. still, i'm not sure how rare it was, or how you would be unaware it existed if you were a fan. as recently as two or three years ago (which was already 12-13 years since it was made) it was definitely on youtube.