Agreed on the general plan as outlined in #1200; I haven't reviewed the patch itself.

We generally use sentence case for headings, not title case, which suggests not capitalizing after a colon. We only appear to get this wrong in 6 sections out of nearly 2000:

compatibility.tex:\rSec2[diff.cpp03.input.output]{Clause \ref{input.output}: Input/output library}
compatibility.tex:\rSec2[diff.cpp14.utilities]{Clause \ref{utilities}: General utilities library}
future.tex:\rSec1[depr.meta.types]{Deprecated Type Traits}
future.tex:\rSec1[depr.iterator.primitives]{Deprecated Iterator primitives}
iostreams.tex:\rSec3[fs.op.weakly_canonical]{Weakly Canonical}
threads.tex:\rSec2[thread.req.lockable]{Requirements for Lockable types}

I would suggest fixing those six instead of the proposed change. (Note that the last one should presumably be changed to "\tcode{Lockable}" instead of "lockable".)