I had imagined following a similar structure to what we've been doing with the language design, where the design just has a short list of alternatives, and the actual discussion is kept in the proposal doc.

Did I give a different impression in the discussion? If so, I may have just misspoken...

Or is there something that makes you prefer this structure?

Sorry, this is a detail I heard from you -- I just don't think it came up. I'd assumed we'd continue to document alternatives in the toolchain's design, as has been done in the past. For example:
https://github.com/search?q=repo%3Acarbon-language%2Fcarbon-lang%20path%3Atoolchain%2Fdocs%20%22Alternatives%20considered%22&type=code

Since it sounds like your preference is to use proposals for more, would you suggest moving all the existing alternative documentation out to proposals for consistency? (or is being consistent with the existing documentation good?)

Note too, I think other documentation has included some alternative documents inline.

Not sure there is a lot of value to retroactively moving stuff around, particularly for the toolchain.

But we should pick the pattern we want going forward and try to be consistent with that maybe? I've been happy with the approach in the language design where the alternatives are just listed in the design but discussed in the proposal in question. But I'm open to other organization if that's better.

WDYT @zygoloid ?

It's really not much extra work (and I think it'd be useful to move in order to avoid confusion with future contributions, especially since otherwise we'll end up with both styles in the same document) -- #6716