Last year, Google began blocking the installation of Chrome extensions that weren't from the Web Store. However, it didn't affect Chrome's dev channels. Starting today, that changes.
Google claims that, since disabling non-Web Store extensions, they observed a 75 per cent drop in requests for support uninstalling unwanted extensions. As a result, today they will begin blocking extension sideloading in the Windows dev channel, with Mac soon to follow, and all other channels in July.
There are still ways to get around this block, however. Just like before, you can still install extensions manually through Developer Mode. They will be disabled every time you restart unless you go through a rather annoying process, though, which still makes this a pretty big hit to Chrome users. Hit the link to read more.
Continuing to protect Chrome users from malicious extensions [Chromium Blog]