Why?
A comment would help.

Dict[bytes, bytes]?

sha1 (bytes): content id

data (bytes): content's raw data

remove irrelevant fixme.

You mean irrelevant for the in-memory usage (test) i guess.

Explaining why i ask.
For the initial storage, as we starting developing paginated endpoints, i'm wondering if it would not be better to align the endpoint as well. If it's possible that is (i'm not sure we enforce an order for example).

That would be out of scope for that diff ;)
The question might be worth discussing (the conclusion could also be to drop the conditional altogether ;).

Can you please clarify what you mean by 'enforced with multiplicity'?

Can't we skip those?

Pushing filtering on the endpoint consumer feels bad to me.

(and if in the main storage there is such a case, that's an out of scope issue, an issue nonetheless).

I never said it before.
We are growing inconsistent with the formatting.
That seems like nothing but that can end up doing some irrelevant noise in commits.

So this should be avoided.

Naturally, that's up for discussion but in the mean time, that'd be great to align to current conventions.
So. in this case, ending character aligned at the beginning of the initial keyword.

return {
    'contents': matched,
    'next': sha1,
}

As per tool, that'd be great not to bother with this and have some tool doing that for us.
@zack mentioned black 'recently' but we never (re)acted on that matter...

Same as a previous comment, prefer a single return statement when possible/not too obfuscating. Here, something like:

from itertools import chain, islice
next = None,
contents = []
sha1s = islice(self._sorted_sha1s, 0, from_index)
it_contents = (((sha1, v) for v in self._content_indexes['sha1'][sha1])
            for sha1 in sha1s)

for sha1, key in islice(chain.from_iterable(it_contents), limit):
    if sha1 > end:
        break
    contents.append({
            'data': self._contents_data[key],
            **self._contents[key],
            })
else:
    next = sha1
return dict(next=next, contents=contents)