I was reading that you have to have the fabric seasons mod installed on client to get the season colors to display and that if the mod is on a server but you’re running a vanilla client the colors don’t apply. Would it be possible for the mod to send a dynamically changing resource pack that gets applied when you join a server? Like after a server start or restart the mod updates the color palettes of leaves, grass, etc to match the current season.

For vanilla compatibility

but why tho, if you're joining a modded server you probably need other mods anyway

again, why would a mod need vanilla compatibility

I have friends that don't want to deal with installing mods (they don't have the technical knowledge to install Java, find the .minecraft folder, etc) and having server-side mods and a vanilla client is just easier for some; thus the use case for such a mod exists.