Let me give a concrete example to make it more clear what I exactly mean.
I have two drafts of C++ standard: N4296 that is quite old now and more recent revision N4750. There are some subsections that I am interested in, e.g. [unord.hash]. Version N4296 requires from
std::hash to provide two nested types
result_type, but this requirement no longer present in N4750.
How can I find the revision, where this requirement was removed and the motivation for it?
This can actually be kind of hard.
There's the working group's mailings.
There's the standard's source whose history can be examined using Git tools and their friends. The commit log in theory should be useful — though I recommend noting down the name (e.g. a word like "N3690") of the Final Draft for each standard so that you can recognise it in the tag list.
This is your best bet if you're literally looking for the specific revision where a change was introduced.
When trying to determine in which standard the change was introduced, personally I tend to just open up individual standard documents and do my own visual bisection. This works well if you know where the feature's wording is located in the standard, and if the wording is mostly compartmentalised in one place, though it can still be time consuming.
For motivations you'll be looking for the original proposal paper. If you manage to find the draft revision where the change was made, hopefully someone will have cross-referenced the name/ID of the proposal.
I also find that Google gives some good results when searching for this if you already have a vague idea of its contents: e.g. "C++ if declaration definition while for consistent proposal".
And, if you don't mind non-authoritative sources (which should nonetheless be reliable), there are usually Stack Overflow answers that track changes between C++ standards, with links to the relevant papers. For example, this answer to "What are the new features in C++17?", which references the changes to
std::hash that you mention.