I hate to say it, but the standard for C1 escape codes, ECMA-48, does list "\e[21m" as "double underline". I can see where one might think it does a "bold off" but alas, that isn't in the standard.

The standard also has a lot of redundancy, and some omissions (getting the size, but I think it was standardized long before TTYs were resizable).

A standard is a good idea, but I think ECMA-48 was a draft that was put into production too quickly.