Like envelope budgeting?

As much as the world should loath it. Crypto?

wallet per folder.

Depending on what the money is for, this is exactly how I use CDs.

With my bank, I can create virtual savings accounts within my account and I can distribute the money as I want. So a "folder" for long term savings, another for emergencies, another for traveling, etc.

Buy Bitcoin?

thats what cryptocurrency is. just manipulated in different ways than normal currency.

Do US banks have limits on how many accounts you have?

I think you run out of numbers at 99 here in NZ but otherwise most banks will let you have as many basic everyday or savings accounts as you want.

Edit: BNZ says there's a limit of 25 transaction accounts for them.

I'm not in the US. But mine are like sub accounts within one savings account out of convenience. I could open more, but I have no idea of there is a limit.

What’s holding you back?

Most people use envelopes for physical cash and virtual accounts for online, but nothing is stopping you from putting cash into folders if that's your preference.

Oh because you don’t play your old CDs anymore you just put some money in the CD case.

Yep, I went back to Betamax.

Does your bank not offer this as a basic service? I think most banks do.

You can assign incoming and outgoing transactions into folders and do stuff like assign repeating transactions automatically to a folder and set targets. So you can make a folder "Saving for a new car" and occasionally add some money to that. Or create a folder "Rent money" and make sure it's topped off and rent payments always go out of that folder. You can setup so a part of your wage automatically gets assigned to that folder for example, so you can easily see how much you have left after assigning money to stuff like rent/mortgage and a food budget for example.

I thought all banks offered this, but it might be different in other countries, or perhaps a paid service?

But even if the bank doesn't offer this, people used to do this in spreadsheet software or dedicated software for years. And even before that, a lot of people used to do this kind of stuff in a little book. In a lot of languages there is even a name for this called Haushaltsbuch in German and Kakeibo in English (from Japanese).
I used to do it in Lotus 1-2-3 back in the 80s, later in the 90s I think I had Microsoft Money which worked pretty good, but a spreadsheet was more flexible.

So nobody is stopping you, you can just do this.

OP wants to store Money, not worthless bullshit

I keep track of money with GnuCash.

There are accounts in there for my checking and savings accounts, credit card accounts, mortgage, car loan, etc.

I also have accounts I created for things that don't exist outside of my GnuCash file. Accounts for money set aside for college payments for my kids, accounts for saving for vacations or whatever. The money in those accounts is "really" in the savings account, but since I don't look there when I'm checking what money I have to spend, it is left alone. So, for example, the bank might say I have $1000 in savings, but GnuCash says it's $200 with $800 in a vacation account.

Some US financial institutions offer similar services. Mine doesn't, but I know I've seen ads for it like it was some cool new technology a few years ago.

Yes, although I'd like to still be able to use a card. Unfortunately you currently have to pick a bank based on whether their mobile app has folder functionality, which isn't a very good criterion to be forced to choose a bank by.

How large is the hurdle for founding one

You could use budgeting software, like ynab.com it also uses the envelope system and supports linking your bank account

Where I used to live in the US, most credit unions had this as a free feature. Not a thing where I live now in Canada, though :/

The price of bitcoin right now is 81k USD.

None. I just click a plus button on my banking app. I can also easily rename them, change the icon, set saving goals and delete them.