They also can vanish because of restructuring, either of its content, or because of the greater degree changes.
This happened a huge amount over my undergraduate degree,
as my university did a near complete overhaul, changing many 4 year bachelors degree's into 3 year bachelors + 2 year masters, and adding significant extra cross disciplinary requirements.
2 units may have there content reorganized into 1, or 3 units into 2 etc.
or unit might be appear to be cancelled, but is actually being deffered as it's position in the course structure has changed.
For example, something might vanish for a few years, if it was originally a 2nd or 3rd year undergrad elective, but became a 5th year post-grad elective.
Which may result from its pre-requisites moving around.
In one of the cases where this happened to me, even the faculty concerned thought the unit was being cancelled for good -- possibly because of the huge mess communicating is, or possibly because it was infact cancelled, only to be brought back as a separate decision, after seeing how it would fit into the new structure.