I Think There is no such specific site for getting such information but i usually get such information through blogs and different people who gives latest and greatest information. For example
https://www.androidpit.com/android-5-0-lollipop-phone-update-news
or like Motorola has this site for update check
https://motorola-global-portal.custhelp.com/app/standalone/country-selector/software-upgrade
i think this type of site may be with different vendors so you can find the latest update they have got.
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solved Latest available Android OS version for different devices [closed]