As far as Markdown-enabled textboxes on GitHub are concerned, the Strikethrough extension is underspecified. In general, it seems to have some of the same left-flanking/right-flanking rules. I haven't looked at the code yet, but here are some examples. According to the spec:

Strikethrough text is any text wrapped in tildes (~).

However, we can find a number of counter examples:

A "right-flanking" ~ cannot open text, nor can a "left-flanking" ~ close strikethrough

The "multiple of 3" combined delimiter rule seems to apply

From the last sentence of rule #9.

The shorter span rule seems to apply

From rule #16

The "links group more tightly" rule seems to apply

From rule #17

I'd be happy to send a PR.