Custom render textures were introduced with Unity 2017: https://docs.unity3d.com/Manual/class-CustomRenderTexture.html

While these are in no way a replacement for the Graphics.Blit proposal, they still open up a lot of interesting things that can be done with shaders.

Custom render textures seem to work fine in worlds, but they do not work correctly on avatars. They work correctly on the first avatar load, but they seem to freeze whenever you switch avatar or join a new instance. This is probably related to the avatar being cached.

The only way to get the CRT working again is by restarting the game.