Grab the Wurst-Client v6.21 MC1.12 OF file from the official Wurst 6.21 update page .

Note: If you are looking for more recent updates, the Wurst team is actively supporting newer versions like Minecraft 1.21.x with the v7 series. Wurst 6.21 - Radar, Realms, Better GUI - WurstClient.net

You can now use Wurst features on Minecraft Realms for both the standard 1.12 version and the 1.12 OF build.