Using Chromium 53, I get on flash applets: `Couldn't load plugin`

This is because on October 2016, Chrome 53, Chrome stopped bundling the Pepperflash plugin. Ubuntu followed suit and disabled it. Here is the official PR from Ubuntu. There are old flash packages in the repo, but they do not work for Chrome:

• pepperflashplugin-nonfree - Pepper Flash Player - browser plugin (this is the Google hosted Pepperflash which they stopped hosting)
• flashplugin-installer - Adobe Flash Player plugin installer (this uses NPAPI which Chrome doesn't support)
• browser-plugin-freshplayer-pepperflash - PPAPI-host NPAPI-plugin adapter for pepperflash (this isn't needed because Chrome natively supports PPAPI)
• flashplugin-downloader - Adobe Flash Player plugin installer (transitional package links to flashplugin-installer)

Ubuntu Way

First we make sure we have the partner repository installed. If we do, this won't do anything. Then we install adobe-flashplugin.

sudo add-apt-repository "deb http://archive.canonical.com/ubuntu $(lsb_release -sc) partner"
sudo apt-get update;
sudo apt-get install adobe-flashplugin;

Now you can go meet Hudson he Muskox on like the last remaining major site to do video distribution in flash (evil eye for Slate).