Hidden Accounts and Loans are listed in Property and Debt Menu
Found at:
For starters: the op provides no information about the Quicken Edition/Release being used. I tested in Q2020 R26.17 and Q2020 R31.8.
Also no detail was provided about how the account was allegedly "hidden". That's important since there are three options that could be thought of as "hiding" an account.
Only one of the account options that relate to "hiding" an account will produce the result the op desires - and there is ZERO reason to believe either of the other two options will hide the account from the dropdown the op is looking at.
To hide a loan account from the dropdown found at the menu selection, Property & Debt > Property and Debt Accounts, you need to select "Keep this account separate ...." on the Display Options tab of the Edit Account Details dialog for the account in question.
Too many users jump to conclusions, and too many responders do the same.