And in true Microsoft form, it's Complicated.
A little over a year ago we learned about the Spectre vulnerabilities, and suffered the consequences, not of cybercriminals exploiting those vulnerabilities, but because the software fixes cut CPU performance significantly. Google had an answer that it freely shared with the world, called Retpoline, which it used to prevent it's cloud servers from suffering the large performance hits plaguing Microsoft's cloud. Briefly it's a way of doing things in software so there's no need for the Spectre fixes that cost performance. Well now Retpoline is coming to Windows -- it's the how that's complicated.
Retpoline won't work on all CPUs, and future CPUs may not need any Spectre-related fix -- while Retpoline is much faster than existing fixes, it's still not as fast as no fix at all. Microsoft's solution also works with drivers, which may or may not be compatible. So Microsoft decided to develop & ship Retpoline *capable* code [files], and let Windows determine if it should be turned on or not. That code is included in yesterday's patch for win10 1809, and in Insider builds since 18272, but only those insider builds have the feature turned on -- Microsoft is going to slowly roll out activation to PCs running 1809, presumably so that they can watch for problems.
The link below is to the discussion on Retpoline, and if you make it to the end of the blog, there's a link to a PowerShell cmdlet to see if Retpoline is enabled or not. That linked page does not include anything about Retpoline -- it just show you how to install & run the cmdlet -- but if/when you run it Retpoline's state is included in the results. At some point in the future some folks may figure out & post how to turn Retpoline on, but otherwise you'll have to wait or run an Insider build. There's finally an Insider build released for the Slow ring, which is easier to live with since you don't have weekly win10 version updates, but that's currently not a great solution since that build has big problems with gaming.
techcommunity[.]microsoft[.]com/t5/Windows-Kernel-Internals/Mitigating-Spectre-variant-2-with-Retpoline-on-Windows/ba-p/295618