Chromium + Debian Stretch 9 - Flash flash player was blocked because it is out of date / has no installation candidate

On Debian Stretch 9 using Chromium I got the warning "Flash player was blocked because it is out of date" every time I visited a site using Flash. I tried to disable Flash, which failed.

I also tried to install flashplugin-nonfree and pepperflashplugin-nonfree using apt-get. But both of these installations failed with "<package name> has no installation candidate". It turned out that something here is not quite ready for usage as the packages have just made their way to the testing phase according to sources online.

What I did to get Flash to work was the following:

Install flashplayer-chromium to get dependencies - this also installs an old version of the plugin:
sudo apt-get install flashplayer-chromium

Download Pepperflash from Adobe: https://get.adobe.com/se/flashplayer/

Extract the contents on the desktop to a folder named pepperflashplugin-nonfree

Move this folder to /usr/lib/

Open /etc/chromium.d/flashplayer-chromium.conf and edit the lines with flash_library and flash_version, so they point at the newly installed plugin in usr/lib/pepperflashplugin-nonfree/:

flash_library="/usr/lib/pepperflashplugin-nonfree/libpepflashplayer.so"
flash_version=$(grep '"version":' /usr/lib/pepperflashplugin-nonfree/manifest.json | cut -d\" -f4)

Updates

Once in a while Flash gets updated and when it does, then you will get the disturbing error again.

So you need to re-download the tar.gz file from https://get.adobe.com/se/flashplayer/ and extract it to /usr/lib/pepperflashplugin-nonfree or move the contents of the downloaded file there. Then it should start to work again.