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.