We use two distinct styles for multiword index entries, both pervasively:

• \indextext{hello world}
• \indextext{hello~world}

What is the purpose of the tilde? Does it have any special meaning in the index, or is it just a non-breaking space? In the latter case, why are we doing that? There seems to be no value in trying to force a particular (probably poor) line breaking decision on the already constrained index.

What's turning this question from a mere curiosity into a serious issue is that if you mix the two styles for the purportedly same entry term, you get two separate index entries, and moreover, the tilde collates differently from the space, producing a surprising index order.

Unless there's a good reason against it, I would recommend killing all those tildes.