When an edition comes out, there is always a lag before all of the mechanics from the previous edition(s) catch up in the releases. Some mechanics are simply never updated as there is no longer a place for them in the new edition.
Removing a specialized class or option in D&D isn't a big deal - there will be something else and there is a disconnection between the mechanics and setting. This makes development very easy. Earthdawn is a special case because the mechanics are directly incorporated into the setting. Removing a discipline is problematic because they are actually a thing within the setting.
There are 15 disciplines from Earthdawn Third Edition (not including Cathay) which have not been updated. The good news is many of these are better handled by slight modifications to existing disciplines than a brand new discipline of dubious design and support.
Below is a list of the expanded disciplines which can easily be adapted to Earthdawn Fourth Edition (ED4) with existing disciplines. I suggest treating them as a specialization of the parent discipline if there are any modifications which need to be made (e.g. Air Sailor [Boatman]). Also, unless there is a pressing biological reason (e.g. windling disciplines and Taildancer), there are no longer any racial restrictions.
- Boatman: Use Air Sailor as the base. Change Air Sailing to Sailing or Riverboattery as appropriate. Exchange Wind Catcher for Swimming; treat the talent version of Swimming the same as the skill.
- Messenger: Use Scout as the base. Exchange Creature Analysis for Book Memory. This is not the best equivalency, but it largely fits with the appropriate talent options. At the end of the day, this is an entire discipline dedicated to delivering the mail. Kevin Costner would be so proud.
- Outcast Warrior: Use Warrior with no changes. Yes, this is a hybrid of Sky Raider and Warrior, but the combination has little internal synergy or consistency.
- Songsmith: Use Troubadour as the base. Exchange Performer for Artisan if the adept is an artist, rather than a performer, and allow Emotion Song to function through their artwork at its unveiling.
- Taildancer: Use Swordmaster as a base. Exchange Speak Language for Swimming.
- Winddancer: Use Troubadour with no changes.
- Windmaster: Use Warrior as a base. Exchange Unarmed Combat for Stealthy Stride.
- Windscout: Use Scout as a base. Switch the positions of Climbing and Avoid Blow.
- Woodsman: Use either Archer or Scout as a base with no changes.