Oh The Trees You'll Go is labelled serverside only on Modrinth, but is required to launch Biomes We've Gone on client, as it needs it for the singleplayer integrated server. This was noticed while using packwiz, which only installs fully serverside mods on the server. Labelling the mod "Client or server" would work better.

No this is correct. The mod can be server side only if the generation code is on the server side only. Since BWG ships both the generation on the client and server side it'll be required for it.