Okay, I've finished implementing the ability to hide items in the purchase screen. It's an option in the Advanced Mods list called "Hide Purchase Items". As you hide items, an internal list (really a map) is managed that is saved and restored into the save game file. It's backwards compatible with save game files that don't have the hidden purchase item list in it.
Users just "right-click" on the item in the purchase screen to "hide" it, and if they want to "unhide" it, they can go the the "Hidden Items" category and right-click on the item again.
And yes... if you select another category other than "Hidden items", the hidden item will not show up there even it it belongs in that category.
Here's a link of the hiding in action:
Everything is checked into my oxce3.5-plus-proto branch on GitHub if anyone wants to test it.
https://github.com/jgatkinsn/OpenXcom/tree/oxce3.5-plus-proto'
Let me know if this can be improved. Would this be better if it was called "Ignore Items" instead of Hidden?
Again, the premise behind this feature is to cut down on the clutter of all the items in the purchase screen you no longer care about because of obsolescence over the course of the game. This is particularly helpful with the mods with tons of items like XcomFiles and FMP.