Over the past year, numerous forks of Paper have popped up. A fork refers to software that modifies or extends the original software, in this case, Paper. Forks of Paper generally add further "patches", or changes, on top of the ones Paper already provides. The question, though, is should you use a fork of Paper over Paper itself?
It's essential to understand why many of these forks exist. In 2018 a project called byof was released by one of the Paper developers, electronicboy. This project made it very easy to create forks of Paper, leading to slow growth in the number of forks and fork developers.
One of the most popular types of forks is experimental performance forks. Developers generally run these forks to squeeze as much performance out of the server as possible. While this is similar to the goal of Paper itself, Paper requires that the changes are well understood and proven to be stable. These forks allow developers to trial more experimental performance changes that may not be safe. The developers may then potentially contribute the changes to Paper in the future. The most used performance fork is Tuinity.
In most cases, large servers create their forks to more easily make invasive changes in Minecraft code to suit their server better. For some more complicated changes, using plugins can be more complex than just modifying the game code. These forks are often very specific to a single server, so they rarely make sense to use. If you're trying to make a mechanical clone of a server, it could theoretically make sense. Otherwise, it wouldn't make sense to use these unless you need those changes. Some well-known examples of these include Empirecraft and Purpur.
There has also been a growing market of paid forks. These often boast about significant performance improvements or making various game mechanics asynchronous. These forks have historically not lived up to the performance claims, and in some cases, been extremely unstable. While growing in popularity, these should seldom be used over one of the more well known free forks, such as Tuinity.
Due to many available forks, some developers have decided to try merging patches from many forks together. Due to some of these patches' complexity, it's often very unsafe to use forks that do this. They're known to break the Minecraft server and various plugins consistently. A typical "everything" fork is Yatopia.
Using these forks have several downsides. While the more significant forks, such as Tuinity, are generally acceptable to use in most circumstances, other forks can cause unnecessary trouble in running a server.
As these forks often have much smaller communities than Paper, it's much harder to get support for less common issues. The Paper community cannot and will not support servers running forks of Paper. Without support from a large community, you're often on your own when you need any help.
As these forks modify the Minecraft server in often significant ways, they can sometimes break plugins. Plugins usually won't support these forks of Paper, too, due to their propensity for causing plugin breakages. In many cases, the plugins cannot easily fix the problems, as they exist in the fork itself.
When using Paper, you've got a highly skilled and community-focused development team. For many of these forks, the developers are just starting or are just in it to make some quick money. A notable exception to this is Tuinity, which experienced developers create.
Overall, using these forks is often a bad idea. If you're using these, you need to have a solid understanding of how the Minecraft server works. While some forks such as Tuinity are significantly safer than others, they're still considered experimental, so they should only be used when necessary. In most cases, using Paper is the way to go.