So the answer is clearly yes and no, and yes but and no but, and yes and and no and. Perfectly clear and simple.
It sounds like a silly response, but when you start with a premise using a word as nebulous as 'relevant,' it's not wrong either.
I mean, fewer people are reading Andre Norton or Margaret St. Clair than were in 1970, 74, or 77-79. Certainly fewer people are reading it and discovering A/D&D (in whichever order), and deciding
'I will put stuff I picked up from this literature and put it into my game.' At the same time, there is still plenty of stuff from this material in the game. So it really becomes a rabbit hole of 'what do you mean by relevant?' and 'what does something have to do to be relevant?'
My position: I drink broadly from the well of TTRPG options. I have active/recent games that are using D&D/D&D-alikes, Traveller/Cepheus engine, Powered by the Apocalypse/Forged in the Dark games, stuff from the Champions/GURPS/Heroes family line, Monty Cook's stuff, standalones and homebrew. If I choose to play D&D at all, it is because I want something of the "D&D feel." That's going to include some of the old pulp magic and Tolkien energy and other things I pulled from Appendix N -- either directly or from my time playing a BX/BECM/AD&D mashup from when the game was more directly influenced by those works. So yes, those books do still have some relevance for me.
Here's the other part: this comes up for me whenever people (inevitably) debate whether older D&D was heavily Tolkien influenced or whether it was (as Gygax at least sometimes claimed) really more sword&sorcery/sword&sandals influenced, just with elves and dwarves and ent/balrog analogs because people would expect them. The answer to me is that it wasn't really either -- it was a team-effort treasure-hunting game based off a miniatures wargame* with a play-pattern formed when it was discovered experimentally that that's the part of a scenario players were actually interested in. It was that first and any given fantasy literature or genre second. So the question of whether specific fantasy literature is 'still' relevant today is complicated because it often was only relevant back then at a relatively light level unless you specifically wanted it to be so.
*the fantasy part itself something of a mild overlay/mod of a general medieval wargame for those who might be interested.