@biggersworldThere is a possibility that some of the advice is dated here, yes, simply because the Online Store has been a continual work in progress since its launch. Square has been loading the site up with many new bells and whistles at quite a breakneck pace.
That said, the category availability is still there. When you create a category (or modify an existing one) and scroll down past the item list you should see a screen like this:
Choose "Only certain days and times" and you'll see a screen like this:
Here is where you can set the available hours for that category. You can also have multiple times per day if need be.
From what I can tell, the design of the site (multi page versus single page [used for restaurants for pickup/self-serve ordering]) doesn't matter: category availability will work in any format.
More information can be found in this help file: Set Up Time-Based Categories in your Square Online Store
Please note that having category availability set does not mean that the items in that category aren't visible on off times. All items are still shown, but your customer will only see "Available later" instead of a way to order the items.
