The person who posted this question gets it. They understand that the real cost of working in spreadsheets isn't the heavy lifting, it's the repetitive friction of doing the same simple thing over and over. Copying a custom format string into a new workbook isn't hard. It's just tedious. And tedium is what quietly eats your afternoon.
So here's the honest answer: there is no built-in way to save a custom format so it appears in the dropdown for every new workbook. Not in the current versions of the major spreadsheet tools. That means the workaround they're using, copy, paste, repeat, is the workaround we all end up with. But the fact that this question needs to be asked at all is the real story. A format like `[<999950]0.0,"K";[<999950000]0.0,,"M";0.0,,,"B"` is a small piece of logic that saves time every single time it's used. It's not a formula. It's not a macro. It's just a preference. And preferences are exactly the kind of thing software should remember for you.
The good news is that this problem is solvable, even if the solution isn't obvious. You can store your custom formats in a blank workbook saved as a template, then open that template instead of the default. Or you can keep a dedicated "format reference" file and pull from it when needed. Neither is elegant. Both work. But the deeper point is that this friction is a design gap, not a user error. When someone in marketing knows their number formats well enough to write conditional brackets and comma scaling from memory, the tool should meet them halfway. Instead, it makes them carry that knowledge around in their back pocket.
What we'd like to see is a future where custom formats sync across workbooks the way themes or styles do in a document editor. Save once, use everywhere. That's not a radical ask. It's a basic expectation. Until then, the practical move is to build your own template and make it yours. Do that, and you'll never type `[<999950]` into a fresh workbook again. That's not just a convenience. That's the difference between a tool that works for you and one you have to work around.