Colocating the redirects with the docs content itself has multiple benefits:
Most importantly, it's possible to update the redirects in the same PR that the content is affected
This prevents having to coordinate updates across multiple repos, which can be tricky to do without causing 404s and the like at least temporarily
This also versions the redirects
Only the "latest" version of the docs site has functioning redirects
So if you make TOC changes in another version (e.g. 7.0, at the time of writing), you must (currently) only represent those changes in redirects after that version goes stable.
If redirects were versioned, they could be updated alongside TOC changes and automatically become active once the release goes stable
We could also (possibly) use those versioned redirects to build a final version of redirects that works for the versioned sites, not just the "latest" one
Colocating the redirects with the docs content itself has multiple benefits: