This is a very popular mod, for an exceedingly popular game. Parts of it are clearly based on Meadow, the game, as the title would maybe-kinda-imply, however there isn't any mention of it on the mod page itself. Even in the 'Credits' section.

For anyone reading this who is unfamiliar, Meadow is a multiplayer exploration/chill game in the Shelter franchise, where you play as different animals and explore a map together with other players all trying to open Obelisks (the small Echoes in this mod) by having enough people standing near them. Those then give you puzzle shards that let you unlock different animals, or skins and emotes for your current animal. Speaking of emotes- they're the exact same, except with Meadow animals and not Rain World animals. Meadow has a very distinct artstyle so they're instantly recognizable, even if the symbols and portraits have been switched around. Their layout is also the same. (Meadow also has the quirk where those emotes are the *only* way to communicate, that's part of what makes that game unique).

I don't want to imply that the Rain Meadow devs stole from Meadow or anything, but some proper credit to the game would go a long way I feel. The original game's servers are all but dead right now, after this many years, and Meadow wasn't a very popular game to begin with. But Rain World is.