Originally Posted by SirBC
I used to be able to exclude books based on a tag in Calibre so that books tagged with, for example, "Programming" would not show in the generated catalog. Is it still possible to do that (I'm not seeing that field any more)
You are right in that the field has disappeared - that is because the facility has been replaced by the more generalised case of a Search filter (based on Calibre search syntax which should mean it is reasonably familiar to Calibre users) which in this case you would put into the "Main catalog filter" field.
The use of Search Criteria is discussed here
in the online documentation which details both the syntax and the fields that can be used within filters. It also provides some examples which are relevant to your case. Search criteria are now also used in other places such as for defining featured books catalog section or custom catalog sections.
For your specific example you could use something like:
in that field to exclude books containing that tag. However you can now build up powerful compound conditions to control what books are included/excluded which previously was not possible.