Hello,
As the Pixelmon mod implements more features that allow customization via resource packs it would be nice as a server to be able to query the client for the presence of a resource and notify them of the need a resource pack.

Example use cases:

• Server awards a custom ribbon, client has no resource pack, ribbon UI will just present the default asset as well as "broken" name, title and description.
• Custom textured Pikachu spawns, server lets the client know via a message, client is confused since all they see is a regular Pikachu
• Chest UI using the new ItemUIElement, client doesn't render anything due to the resource not existing
• A trainer NPC using a custom made skin, client doesn't have the resource pack no skin loads

Minecraft is able to recommend clients a resource pack with a built in download, why not use that?
The file size limit in 1.12.2 is simply too small for any pack containing more then a handfull of Pokémon textures, even with the modern Minecraft versions the size limit is still not enough for any server that makes textured content on a semi regular basis.

Why give you a way to pester players?
While I do understand this point wouldn't it be a lot better to notify just players who actually need it when as it stands I can already just notify everyone? People are still able to ignore it but are at least aware they aren't getting the intended experience.

A sidemod should do this not us
Another perfectly fair point but given that Pixelmon is the one providing all these reasons to want players to use your resource pack wouldn't it make sense to be a built in API functionality? Not to mention the fact most of the users of these systems probably won't know how to make their sidemod or even above all, if a player isn't installing your resource pack why would he have your custom sidemod to begin with?

What would be nice to have
My knowledge of the client side of modding is basically null so I'll just assume that a distinction between texture and language assets is required.
Simply having an API method to send the query request that takes in the resource location or the equivalent string and the asset type, once the client processes the request simply sending a packet that triggers an event would be enough

Thank you!