Can I delete a category that is no longer usable? (Q Mac)

Quicken Jasmine said:

You can delete categories by navigating to Window > Categories > Right-Click on the category > Delete Category. 

In the same window, you can also just click the minus icon at the bottom (instead of right-clicking). Quicken will want you that the category is own use, and transactions using that category will be left uncategorized. I encourage to to follow RickO's suggestion to either merge the old category into one you use now, or if that's not appropriate, to add a "z" prefix to leave the category but send it to the bottom of your category list.

There is an Idea thread (feature request) from users to add a feature to allow unneeded old categories to be hidden — like you can currently hide old Payees. It has 149 votes, and you should click on it and add your vote if you've like to see such a feature added. (Click the little light gray arrow under the vote counter in the yellow box.)