Keep a visible archive of old emails in Mail App but remove account from the list

If you are really not using the accounts anymore and never will, you can move their content to archives either "On my Mac" or in iCloud if you have the space. I recommend on iCloud because you're protected against their loss should anything happen to your computer. In both cases click on the circled + on the right of their name in the list of mailboxes, create the folders for their organisation and drag messages from the disabled accounts.

Once all of this is done and you can disable or remove the old accounts permanently.

Just tested to confirm. I dragged a mailbox "test" with a nested mailbox "test 1" from one account to the iCloud account itself or onto one of its own mailbox. The hierarchy was maintained and all messages went along. The difference with dragging a single message is that it makes a copy of the structure and its content, so for a while you have your messages at two locations until you decide that all is OK in iCloud and you can get rid of the old account.

As you've notice the circled + only appears when you put the mouse cursor in the vicinity of "On my Mac", same with iCloud (visible if you have an iCloud mail account and have enabled it in the System Settings under your name / iCloud).

If you want to put the old messages in the cloud the process is not to move the messages in folders "on my Mac" but to create those folders immediately in iCloud.

Once you've dragged the messages to these folders the original folders will be empty (it's a move, not a copy unless you held the Option key). Start with only a few insignificant messages to understand and trust the process. It's basically the same process than with the Finder.