XDG is a systemd standard that's not universally used across Linux distributions. It's most definitely not used in other UNIXy systems like FreeBSD and MacOS. XDG, like systemd, likes to believe it's universal but it's not.
In XDG's favour, we can say that it is just a written convention that people are free to follow or not on a per-program and per-user basis. It is not a cancerous growth that tries to borg the whole system.
"I use Ubuntu and develop in full stack JavaScript so everyone else does too."
No, it is not "practically universal". Alpine Linux, MacOS, FreeBSD, illumos UNIX distros, etc. do not follow this imposed "standard".
>Even if there's a Linux distro going out of it's way not to set them
You have this backwards. The systemd standard is imposed on Linux, not the other way around.
>In practice I've never encountered users with a distro that didn't have these variables set.
"I use Ubuntu and develop in full stack JavaScript so everyone else does too."
A systemd standard, we covered that. Great reason not to use it.
>It's absolutely no skin off your nose to adopt it
It is if I have to implement logic to support this shitty "standard".
>beneficial to you and your users if you do.
Poorly thought out and imposed standards are not beneficial to anyone.