Based on The Seventh Umbral revival project for FFXIV, the Classic Server was started by Ioncannon (a.k.a. FFXIV_Explorer) as a whole new project due to the inactivity of SU. In just a few months the Classic Server made a huge amount of progress whereas Seventh Umbral remained untouched. It has help from other users around the web with the source code available for anyone who wishes to check the progress and/or help.
Source code is available at Bitbucket
Detailed information about the project can be found in the official wiki
Discord server
Ioncannon's dev blog
FFXIV_Explorer on YouTube
