Quick A: Use wg21.link

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How to track newer C++ std documents of given topic?

For the newer proposals (ones that start with the letter P) you can use wg21.link redirect service to obtain the latest document:

wg21.link - WG21 redirect service.

Usage:
wg21.link/nXXXX
wg21.link/pXXXX
wg21.link/pXXXXrX Get paper.

wg21.link/standard
Get working draft.

wg21.link/cwgXXX
wg21.link/ewgXXX
wg21.link/lwgXXX
wg21.link/lewgXXX
wg21.link/fsXXX
wg21.link/editXXX
Get issue.

wg21.link/index.json
wg21.link/index.ndjson
wg21.link/index.txt
wg21.link/specref.json
Get everything.

wg21.link/
Get usage.

For example for P0476: Bit-casting object representations if we use wg21.link/P0476 we obtain the latest version which is P0476R2.

In my answer to How does the standards committee indicate the status of a paper under consideration? I go into more details of the WG21 site and what documents you can find there.

Use the everything link for Pre P proposals
If we use the wg21 redirect service Get Everything link we can do a text search for the paper title. So for your example Improvements to std::future<T> and Related APIs we can see the last document is N3857:

"N3857": {
"type": "paper",
"title": "Improvements to std::future and Related APIs",
"subgroup": "Concurrency",
"author": "N. Gustafsson, A. Laksberg, H. Sutter, S. Mithani",
"long_link": "http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3857.pdf",
"link": "https://wg21.link/n3857",
"source": "http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/",
"date": "2014-01-16"
},