But why? The trend towards putting icons into Unicode may be a mistake. Unless it's a symbol one uses in a sentence, there's no real reason to have it in Unicode. Unicode should not be viewed as a standard clip art library.
Your fonts don't have to support the entirety of Unicode. That's why we have font stacks and fallbacks.
"To start the device, press the ⏻ button on the device face"
There, used in a sentence.
"Every possible image" (e.g. an elephant icon in running text) is not.
A standard clip art library that covers universally understood symbols sounds like something that would be very useful.
What is "plain text format" though? If 'text' isn't limited to Western ASCII characters (which it very obviously shouldn't be considering many people use other character sets), then the idea of a text standard should be to encode all the glyphs people use, so "plain text format" becomes a canonical list of all the communicative symbols in all languages. That's what Unicode aims to be.
In my opinion, if they're used for communication, it doesn't seem unreasonable that such a canon of characters should include universal iconographic symbols like the standby icon.