Google has removed the option to switch between Adobe Flash and the HMTL5 Video Player on YouTube for Chrome users.
You have two core options when it comes to playing videos on YouTube. You can use Adobe's Flash Player for that or YouTube's HTML5 video player.
Flash was the format of choice up until now regardless of browser used and the only exception to that was if Flash was not available or enabled on YouTube. If that is the case, YouTube tries to switch to the HTML5 player automatically which may or may not work depending on the browser's support for HTML5 Video.
If you wanted to use HTML5 by default, you can visit the http://www.youtube.com/html5 HTML5 Video Player page to activate it. When it started, it was called HMTL5 Beta but now it is just the HTML5 Video Player which suggests that it is out of beta.
So, that page displays whether you are using the HTML5 video player or not on YouTube.
Chrome users who have visited the page recently may have noticed that they cannot switch to Adobe's Flash Player anymore as there is no option to do so, and that HTML5 is selected by default.
The page opened in Chrome displays what the browser supports and lists that "the HTML5 player is currently used when possible".
What this means is that you cannot switch to the Adobe Flash player anymore if you are using Google Chrome. If you check the page in other browsers you will notice that you still get the switching option between HTML5 and Adobe Flash.
Why is Google doing that?
The most likely explanation is that it has something to do with the company's announcement that it will kill all NNAPI-based plugins in Chrome this year. Since Flash is one of those plugins, it is only natural that it won't be supported anymore on YouTube.
The wording makes it look like as if Flash may still be used at times, but that YouTube will prefer the HTML5 player whenever possible if you are using Chrome.
Is there a way around this?
I have not tested this, but it is likely that changing the browser's user agent may display the option again on the HTML5 Video Player page on YouTube.
You can use a browser extension such as User-Agent Switcher for Chrome to test that.