So for me, getting flash to work in Chromium was important because I have a Slingbox and want to watch via browser. Not having it work with LibreElec led me to try Kodi with both Windows and Linux. Bottom line learned was if you just want Kodi, nothing better than small build and dedicated box. This fix is working on LibreElec for x86.
1) Download and extract the flashplayer plugin for Linux x64 directly from Adobe. I used this one:
2) Extract archive until you see/find the following file: libpepflashplayer.so
3) Assuming you have already installed flash via the configure section of Chromium addon from LibreElec, copy this file to your LibreElec box in the directory: /storage/.kodi/addons/browser.chromium/bin/PepperFlash
4) Now lots of posts doing the above, but here is the oh so simple thing that got things to finally work: Start Chromium and go to the following address:
Check the box "Always allowed to run" and exit Chrome. Restart and you hopefully you will have working Flash in Chromium.
Hope it works and post your observations. Thanks to all others who have taken the time to post...certainly helped me. And thanks to the developers...amazing work that we all benefit from.