We've all wondered it at some point - are custom icons safe to use? Will I get banned for using them? Does EAC (Easy Anti-Cheat) care?
The quick answer is they should be safe. I'm not aware of any issues and thousands of people use NightLight Desktop each day and have used it to install packs around a million times.
The installation of icon packs is simply a matter of placing image files in a specific folder. Dead by Daylight then elects to read them and use them over the bundled icons. It seems unlikely it would function this way if the devs had an issue with it (especially as it was broken and then returned to working like this a long time ago).
(NightLight Desktop greatly simplifies the process of finding & automates the downloading and copying of the icon images. You can of course still do it manually - under the share menu is a button to download the pack zip - but there shouldn't be any extra risk when using the app).
Unsurprisingly Behaviour can't/won't officially condone custom icons, here's a comment from a moderator on the Steam forum (the essence of which has since been repeated by them several times):
Modifying any game files is done at your own risk.
We are not manually banning players for modifying .png files in their game folders, however, the anti-cheat may detect this and ban you.
If this occurs we will not be able to assist you.
To summarize, it's not officially endorsed, but it's also not disallowed & there have been no known problems with it. If you have any questions, feel free to stop by the Discord!