I vividly remember how it took them several months to make Opera display Flash content correctly (at 2x scale instead of a blurry upscaled-after-rasterization mess) on retina displays when I just bought my macbook. Other browsers had it done in a matter of weeks, days if you installed a beta build. And that was 2012, back when flash was an essential part of the web.
Flash was not an essential part of the web in 2012. Maybe, if you had some weird work tool you had to use, some video players on the web but that was mostly it. In 2012 flash for Android was killed and it was very well known that it was going out of the window in the near future.
Many video players were still using flash. VKontakte still had its flash app/game platform going strong, though you did have the option of using an iframe that loaded a web app from your server instead. Flash support was only discontinued there around several years ago.
I used it exclusively for about a year (iirc) on Linux and had only some minor crashes. When they switched to Chromium I switched to Firefox and never had a single crash since. (It's not only luck, I disable all the feature-creep I know of in about:config)
