I downloaded the Event Catalog last week to start filtering, sorting, and searching in Excel; then downloaded an updated version yesterday and noticed that some of the sessions are no longer in the catalog. I expected new events and new sessions to be added, as well as come events filling up, but I wasn't expecting that many events/sessions to disappear from the catalog altogether. Does that happen often? It seems like it would make planning my priorities more difficult and confusing if the things I want to attend are no longer in the catalog after I finish prioritizing them.

It's fairly common as you can see here.

_____________________________________
​​​​​​​Alec Usticke, Fans of Gen Con Facebook Group

Yes, not common but happens.  It is also possible for the events deleted to be re-added later. 

If the number of events are on line with last year we are still missing about 3000.