Unless the list is sprawling and encyclopedic, any kind of "abbreviated" "essential" is defacto gatekeeping.
This is just flatly misusing the term "gatekeeping".
Gatekeeping means actively attempting to limit access, and making a list of some significant influences and/or further reading is absolutely not gatekeeping. Claiming that it is doesn't make it so. On the contrary, it demonstrates that the person claiming that doesn't understand what they're reading, and is applying a completely inaccurate and unhelpful interpretation to it.
The only exception would be if the list of influences was prefaced with text essentially stating "These are the sole influences on this work, and represent the only styles and tones appropriate for it to be played in!", which is actually a thing that has happened! But it's the exception, not the rule - I'm trying to remember what pompous and ridiculous RPG did do that (pretty sure it was between 1997 and about 2010).
But the vast majority of Appendix N-type influence lists openly say
the exact opposite - they point out that they are
not exhaustive, and that in fact, they're merely some examples. To read that as gatekeeping is, at best a failure of basic reading comprehension and media literacy, and at worst, malicious and intentional misrepresentation of what is being said.
You also don't need even need to list actual influences - you can just list stuff that fits the tone and ideas of what you've written.
Even then, is a corporation willing to attach its reputation to authors from a previous century who are sometimes problematic?
I think this is closer to the real reason, but it's still off the mark. It's pretty clear which authors would be "problematic", and you could obviously either acknowledge that, or just skip those. I think the real issue 5E faces is that WotC essentially wants the new 5E (and indeed pretty much all 5E for the last 5+ years) to be family-friendly and PG (not even PG13, really). But any list of fantasy novels, like any list at all, is going to have some stuff on it that some zealous parent is going to find objectionable. And parents are a million times more likely to read or learn of the Appendix N equivalent today than decades ago. If D&D were aiming at older kids or adults, I don't think this would cause WotC to even blink, but because WotC really wants to get younger kids, not teenagers even necessarily, involved in D&D, it is.