I have a large document that used to be in rich text, but must now be in plaintext. The document previously utilized nested lists to organize information.
Is there a "best practice" for replicating nested lists with plaintext?
There isn't a "best practice" to point to. The type of information you presenting could guide what you want to use.
Personally, I just use an ordinary
It is simple, to the point, doesn't distract from the content, and is a standard character that is not going to cause problems when you pass the file around to different people on different machines (using different operating systems, and different programs to read them).
If the data is more structured using '1.', '2.', etc... and indenting to 'a.', 'b.', 'c.' may be more appropriate.
Whatever you choose, you should not select any special character. The fancier your try to be the more likely you are going to run into trouble (such as database issues, as you point out in a comment). Here is everything you have to choose from using standard ASCII: