Color must never be the only indicator of the status, hence some additional part needs to be added. However, instead of 0/1 which would need localization for languages not using arabic numbers, I would rather encourage this (quick Paintbrush hack of image from T134795#2356626):

I see what you mean by adding the icon, and it would surely help. But color isn't the only indicator here, contrast and position of the handle are there too (same as iOS, I guess). Does iOS now have those indicators by default, or only when an accessibility setting is enabled?

In T140812#2477558, @Danny_B wrote:

0/1 which would need localization for languages not using arabic numbers

This is already built into MediaWiki. Icons would make this a much heavier treatment and I don't think would be ideal.

In T140812#2767360, @Prtksxna wrote:

Does iOS now have those indicators by default, or only when an accessibility setting is enabled?

Only as an accessibility feature, though I have them on permanently for sanity reasons. :-)

Youtube does something kind of like this with an icon now. Note the toggle in bottom-right corner.

Starting from the original title of this task,1 and 0 are also not necessarily universally understood, as it's of very tech-centric origin.

If at all, a checkmark seems more like an option with all the given negative implications:

• crowed interface,
• still not fully clear what is the “on“ and what the “off“ position of the toggle grip

Code used:

&.oo-ui-toggleWidget-on {
    …
    background-image: url( themes/wikimediaui/images/icons/check-invert.svg );
    background-position: 2px 2px;
    background-size: 45%;
    background-repeat: no-repeat;
}

Same applies to codex