> Without spending too much time on it, it seems to me there are s number of issues here:
> 1. What URL schemes do we have and want to continue to support?
> 2. What things do we want to show up in search engine results?
> 3. How do we make that happen?
> 4. How do we maintain this going forward?

1. unchanged from today is good: let's not break things.
2. agree too: just the most current 3.x maintenance release
3. By adding links from old releases to new releases, go new releases get weight (from a search engine point of view) and should rank better. Also by checking if the canonical header is configured properly on all releases.
4. I don't think hiding old pages from search engine is a good idea, it's lying to them. I'd prefer the way of linking from old to new, which is both user friendly (a) and search engine friendly (b).

a) A nice text like "This is the documentation of an unmaintained version of Python, [switch to the latest version]" will not only help people landing from this old page from a search engine but also from favorites, links from articles, ...
b) Search engine compute the "weight" of a page by adding weights of pages linking to it. Our old pages are linked from a lot of articles and various sources, they have a huge weight. Linking from those pages to our new pages will forward a part of this weight to our new pages, ranking them a bit better. Which is I think better than hiding the old pages from them and loosing the weight they accumulated (risk: get old blogs articles rank better than new official documentation).

About the canonical, I'm having an issue opened indirectly about it (https://github.com/python/docsbuild-scripts/issues/51), by rebuilding old releases we could upgrade their headers and properly set a canonical link, typically from /3.4/ to /3/, like we're currently having from /3.5/ to /3/.

I have an opened issue about handling the /release/ directory from docsbuild-scripts instead of it being a manual step in the release process (https://github.com/python/docsbuild-scripts/issues/48).

I opened an issue in python-docs-theme about displaying a message pointing from old releases to new releases https://github.com/python/python-docs-theme/issues/24 mainly not to forgot about it.