Paper 1.21.4 + viaversion + the latest build of geyser works fine for me for 1.21.70 players, but i don't think viaversion is making any difference since it doesn't support. The 1.21 update stable paper and velocity 1.21 builds have been released! The 1.21.3 update paper and velocity 1.21.3 builds are out of the experimental phase!
The instructions are contained in the content of what's written on the paper. Paper contributors hardfork affects contributions to paper dramatically, most of it for the better. As always, backups are absolutely mandatory.
After upgrading your world to 1.21, you cannot. I looked around and couldn't find. After upgrading your world to 1.21.3,. 'on the paper' is on (the surface of) one sheet of paper.
Has anyone found a way to make spigot's plugin annotations work on paper servers as a replacement for bundling a plugin.yml file? That's a harder question than you might think! Something is 'in the paper' = 'in the newspaper', because a newspaper has many sheets, like a book, so things are. The instructions are written on the paper.
As always, backups are absolutely mandatory.