How did you collect this information? Manually by checking all compilers? By writing tests? By parsing compilers documentation?
Then I wrote a bot that watches all known (and most importantly, reliable) sources for changes, that then notifies me. The data itself is kept in very simple yaml files. So whenever something changes, I verify and update the info accordingly (The site itself is then regenerated and uploaded automatically).
For features that are not fully implemented yet, or where the vendor does not provide any information (e.g. Apple's conformance table), I set up a conformance test suite on my machine that runs most of these across all toolchains.
Please note that I won't be the only person that maintains this site. I'm in the process of open-sourcing and automating most of it, so that everyone can contribute via GitHub. It's just that keeping up with C++ is part of my job, and also a personal interest of mine. So I do this as a "labour of love".