Apple doesn't share why they remove videos, but they have certainly removed older WWDC videos, and it seems like it happens most often when the subject matter is out of date. WWDC videos are usually fine, even for concurrency, they'll just lack nuance as time goes on and new features are introduced, or new defaults are used.
In the future I hope that the longer form documentation proposed in another thread could be the ultimate source of truth for language features, updated by the community over time.