Hi, I would like to ask if this is a client only mod or if it also brings features / improvements for servers?

It's client-side only

Could you please add a note that it's client-side only to curseforge.com and the readme?

I *suppose* the clarification can be in the form of

-Culling is when you don't render things that the player can't see, to improve performance
+Culling is when the client doesn't render things that the player can't see, to improve performance