If a player has join from version 1.8, the plugin offers to install a texture pack suitable for this version. If the player join from 1.13, it offers to install a texture pack suitable for this version. The same with the other versions. The client version can be obtained from ViaVersion or ProtocolSupport. I propose to implement this feature in your plugin, it is suitable for servers that run on several versions of Minecraft.

This would be interesting and not too hard to do if there is a way to detect each players' client version.

I'd argue it would probably more appropriate for a more specialized resource pack plugin.

(This would also not work when running viaversion on BungeeCord)

Hm- wouldn't that be the main use case? What other way is there to have a mix of players on different versions?

I think if this was really needed, server owners can now set it up themselves using alternate_resource_packs - it won't be automatic, but players can set their desired RP and I think that's probably good enough.