It looks like there are a few missing topics in the precalculus course, namely:

1. Videos on how to graph polar coordinate equations, such as roses and limacons. This has been requested multiple times on this forum. Sal actually says that these will be covered in the next video, but there is no next video.

2. Word problems involving parametric equations. Sal only talks about parametric equations but none of the applications.

3. De Moivre's Theorem and nth roots. This could fall into precalculus or calculus, but is usually taught in precalculus.

There might be more missing videos, feel free to reply in the comments. Hopefully these gaps will be fixed soon!