As of today, Windows 8.1 is out of offical mainstream support from Microsoft. The deadline has passed which means most of us still running the last-gen operating system won't see updates or be able to call support.
Of course, the code crunchers in Redmond might tap out the odd update, just as they did last year for Windows XP following the WannaCry outbreak. If you're eligible for extended support then you've still got another five years to go before you're totally out of help.
Windows 10 is a little tougher to keep track. With two major releases per year, it's not a version number or service pack you need to keep track of, but a specific build. Build 1607, which was released in August 2016, is tentatively scheduled to leave mainstream support in March this year, with Build 1703 getting the support bullet in September this year.
With Windows 10, the rule of thumb seems to be that each build gets about 18 months of mainstream support. If you stick to an approximately annual update cycle then you should be able to stay supported.