Now for the elephant in the room. Eaglercraft exists in a gray area with respect to Minecraft’s EULA and intellectual property.
Mojang has not issued a DMCA takedown against the main Eaglercraft repositories (possibly due to the effort of reverse engineering vs. copying code), but many forks have disappeared from GitHub. The project persists via GitLab, self-hosted archives, and Discord communities. eaglercraft-server
If you run an Eaglercraft server, you should: Now for the elephant in the room
You cannot join an eaglercraft-server with the official Minecraft launcher. You need the Eaglercraft client. Mojang has not issued a DMCA takedown against
How to set up your own Eaglercraft server (full guide + download)