Hot on the heels of the release of saved reports for Income & Expense a couple of weeks ago, categories can now be hidden on a per-saved-report basis, instead of globally for all reports as it did previously.
When you have any hidden categories, a menu item that opens a management modal appears below the toggles in the sidebar. You can adjust what categories are hidden within this, and then either view those changes temporarily, or save them against the report that you’re viewing.