Minor During the 1.13 update, the limit on the length of plugin channel names was removed (source). Independent inspection of the source code via Fabric confirms that this restriction is definitely removed in 1.15.
The 1.13-pre7 update to Spigot by @md_5 changed the limit to 32, but did not remove it like in Vanilla: https://hub.spigotmc.org/stash/projects/SPIGOT/repos/bukkit/commits/f8b2086d60942eb2cd7ac25a2a1408cb790c222c#src/main/java/org/bukkit/plugin/messaging/Messenger.java
I encountered this issue when a user of my mod was unable to join a Spigot server, but could join Vanilla and Fabric servers: magneticflux-/fabric-mumblelink-mod#22.
[SPIGOT-5681] Plugin Channel length does not match vanilla restrictions
Mitchell Skaggs
added a comment - I think a maximum realistic identifier would be 128 chars, and the limit should be double that: 256 chars. Yes it's 8 times larger that the current limit, but it's still only 1/128 the Vanilla limit.
Mitchell Skaggs
added a comment - I think a maximum realistic identifier would be 128 chars, and the limit should be double that: 256 chars. Yes it's 8 times larger that the current limit, but it's still only 1/128 the Vanilla limit. 
md_5
added a comment - By design, 'unlimited' length fields are bad.
I would be open to tweaking the limit to eg 64, but I would be against removing the limit or setting it to a large number like 32767
md_5
added a comment - By design, 'unlimited' length fields are bad.
I would be open to tweaking the limit to eg 64, but I would be against removing the limit or setting it to a large number like 32767 
In general, I think any difference in behavior from Vanilla should be configurable. I'm sure that's not possible for everything, but having configuration options for divirging behavior would solve a lot of potential issues with interoperability.