Other formats

In beamer, in reveal.js, and in any other format with no index back-end, marks pass through: the visible text is preserved exactly as written, no index is generated, and no LaTeX or index markup is emitted. The mark itself is still a span, so in a format that can carry span attributes — GitHub-flavoured markdown does, through inline HTML — its class and attribute values travel with it, just as any other span’s would. Beamer slides have no index environment of their own, so a \printindex there would abort the render — and a marked term must never break a document, so beamer is deliberately not an index target rather than a broken one.

Warnings about the mark itself — an empty sub-entry level, a cross-reference with no usable target, a mark with nothing to index — are about what you wrote rather than about any one back-end, so you get them whatever you render to.