So, I'm giving it to you now. This is just smart and logical. You do not need technical proficiency to reason out the risks, or the daily "12yo exploits MOST-SECURIST-EVER IOT device in 20 minutes" articles.
With that said, without public support, and government laws, this is a losing battle.
Equifax had no authorization from you to collect and distribute your financial data, but they did it anyway, and your financial services sent them the information. And of course, not surprised, their criminal negligence, revealed EVERY SINGLE ACCOUNT to hackers. Let's be real, when the news for stupids spins the story in this manner:
Equifax data breach: The number of victims may be impossible to know
https://www.usatoday.com/story/money/2017/09/17/equifax-data...
It means that the number was so great as to be unspinnable, so they went with another narrative, "Nobody knows" like dark matter, it's impossible, our scientists are trying but it's impossible....lol
Just assume all of them. Every last customer. Probably closest to the truth.
All merchants that accept credit cards are placed in a category by the card companies, and every credit or debit card purchase therefore gets this category attached to it. Some places surfaces this information to you, in various ways. I have Chase, and they have a pretty shitty UI for this, but there's something there at least.
So it's not unique to Discover, and it's not them "guessing" or trying, it's based on fixed merchant categories.
Citibank includes this code in the "SID" element in OFX generated by their "OFX Direct" service (i.e. https://www.accountonline.com/cards/svc/CitiOfxManager.do). I haven't seen it in the OFX from any other financial institution though. The codes are cataloged here: https://github.com/greggles/mcc-codes
(though I'll admit the sibling's comment about satire is probably right)
I like your concept of the multiple credit/card style budgeting. That makes a lot of sense. There are even some of those multicards out there with an e-ink display to select different cards. A bank could just issue one of those for a fee where you can select the account (and have a way to transfer money/items around if you select the wrong budget option).
Discover card has a section in your UI that tries to automatically break down your spending by store type (grocery, gas, etc.) but it does require you make all purchases with your Discover card, which a lot of people simply don't take.