This is less of a PSA and more of a "fishbirdthing smacks its face into its fins and laughs until it coughs" thing: whether intentionally or by accident, it seems mojang assassinated the old API involved in whitelisting Minecraft players as a slash command in versions prior to uhhhh 1.20.3 or so. (For those who don't speak Minecraft-ese, that's nearly all the versions. And Minecraft players famously play older versions for a lot of reasons, very frequently.)
How'd I find out? Watching someone try to add a friend to server whitelist so they can fiiiiiinally hop in after making big dewey creacher eyes at them for a mooooooonth.
So the fact that the reddit thread hot tip/answer about why it wasn't working was 14 hours old is giving me feelmotions.
The workaround is your would-be player headbutts the server while the server owner reads the console to perceive said player's UUID number and then manually edits that info into the server's whitelist file. This is not impossible. It is, however, a baffling piece of the API to break without warning or acknowledgement, for every version of Minecraft older than December 5, 2023.