Is there a discord server for cross-league collaboration?
Hi Jayen, welcome to the cross-league forum and thank you for contributing the first post to kick things off! 
There is no official discord server for cross-league collaboration to my knowledge, but there are discord servers for (and perhaps more):
- SPL - RC-SPL
- Humanoid League - rc-hl
- SSL - RoboCupSSL
I agree that Discord groups are appropriate when it comes to close collaboration between smaller groups for package/library development and maintenance.
ps.
The article slack-vs-discord-vs-discourse seems to cover the advantages/disadvantages of the platforms pretty well for anyone wondering when to use what.