It is definitely not safe. The security updates that will stop coming address weaknesses in the Windows code. No third party is able to fix that, because no one other than Microsoft can alter the code for Windows.
Microsoft has said that they will continue to offer some security updates for Windows 10, for up to three years after it becomes unsupported, but you'll have to pay for it.
Now is the time for every Windows 10 user, including me, to make plans for how we will continue to use our perfectly good computers after Windows 10 is unsupported. Nevertheless, Windows 10 still has more than a year of support remaining, and a lot can change in that time.
Otherwise, ChromeOS Flex is a possibility, if your computer qualifies. Linux is another possibility.
Here's some good information from Ed Bott of ZDNet: When Windows 10 support runs out, you have 5 options but only 2 are worth considering