Controlled Folder Access

Windows 10 Fall Creators Update (aka 1709) was released October 17, 2017.

Windows 10 Fall Creators Update includes a number of new features, including a replacement for OneDrive Placeholders, support for Windows Mixed Reality, the ability to more seamlessly connect to Windows PCs from iOS and Android phones and an improved Photos app experience.
ZDNet

I've installed it on a couple of laptops with no issues. I haven't seen any problems with it nor really any new features.

Except...

Windows 1709 has a new Windows Defender capability called "Controlled Folder Access".

A below-the-radar security feature in the Windows 10 Fall Creators Update ... can stop ransomware and other file-scrambling nasties dead.

The controlled folder access mechanism within Windows Defender prevents suspicious applications from changing the contents of selected protected folders.
The Register

To turn it on, click on the Windows key and type "Windows Defender Security Center". Click it and then click on "Virus & threat protection".

Then click on "Virus & threat protection settings".

You're getting warm.

Under "Controlled folder access", slide it to "On" and click on "Protected folders".

You can also add programs to the whitelist.

Here are the folders protected by default:

Add any that Windows didn't choose.

The best write-up I've found is here.

Here's what it looks like in action: