Invisible blocks. Blocks that are invisible, except when you're wearing the provided IR goggles on your head.
This mod was made for 1.16.1, but from a bit of quick testing it works fine on 1.16.5 with Sodium installed, somehow. (It might be because this mod is quite small and doesn't seem to do anything more drastic than having a custom block entity renderer which defers to the default renderer.)
Best paired with mods that draw holograms or something.
Feel free to use this mod for your own maps and/or online conventions.
No I will not port this mod to Forge. If you want a Forge version, this should be reasonably easy to port (there's no mixins, although there is a block entity renderer). Follow the licence and you've got permission.