Early on they were used in some serial and modem protocols, which was problematic when you used them in actual file content. I remember trying to use them in a file, and then when I tried to transfer them, the transfer would stop at RS.

Who is typing out CSVs anyway. You’d either use a spreadsheet tool or do it in code.

From my point of view, CSV is mostly operated in text editors because they are much faster and don't risk to do any unintended modifications.

My editor, and lots of editors now, put spaces instead of tabs when I press the tab key. No reason something similar couldn't happen in CSV modes/apps.

Notepad++ show RS and US as little black boxes with 'RS' and 'US' in. It works well enough.