Get your minecraft_server.1.12.2.jar
from the root of your server, download the last bundled jetty 4.1 version https://repo1.maven.org/maven2/io/netty/netty-all/4.1.68.Final/netty-all-4.1.68.Final.jar
Open your server jar with 7zip and navigate to minecraft_server.1.12.2.jar\io\netty\
open the netty jar file you downloaded and go to netty-all-4.1.68.Final.jar\io\netty\
back in the minecraft server jar delete these 6 folders
now simple copy the same 6 folders from the netty package into the minecraft jar
throw the jar back into the root of your server and you are done!
Not quite sure what you are trying to say, this simply updates the included netty version for issues like:
repeated spam of this and frequent disconnects
NetworkDispatcher exception io.netty.channel.unix.Errors$NativeIoException: syscall:writev(…) failed: Broken pipe
It doesn't touch any Mojang code.
Always take backups, but if you mess this up the server probably doesn't even start and you can just download a clean minecraft_server jar file.
No the client doesn't care, and software like Spigot and Bungeecord already do this on their side. Forge is the outlier in this case.