I tried to get some facts. In 2006 I remember Gnash, it supports most SWF v7 features [1]. One could install firefox and flash in wine [2] (didn't know, didn't bother). And in 2008 there was native Linux Flash Player 9 [3] (too late). In 2010 still no hardware acceleration and no 64 bit support on Linux - in comments [4].
So technical availability was issue just in 2006-2007. Yes, I remembered flash was ads so without plugin it was ads free experience before AdBlock. I liked it a lot. The only issue was "copy to clipboard" buttons, but could work around.
Odd, I explicitly remember using an official version of Adobe Flash in Linux.
I can't seem to figure out when Linux support started, but the archived version of Flash Player 7 on Adobe's site[1] appear to include builds for Linux (as well as Solaris!). The earliest Linux installer in the download has a file timestamp of May 20, 2004.
So, from my memory (I started with Linux in 2000 or so, haven't really stopped.)
There was a flash player for linux, I want to say it corresponded with about version 5 but don't trust me on that. It never really got any updates except perhaps the occasional critical security patch. Meanwhile, the Linux desktop moved on and the plugin got harder and harder to keep going. Things like sound required jumping through a few (extra) hoops to make work, I think it expected OSS but that faded out to be replaced with ALSA, so you had to add some thing to get backward compatibility for that, and it would tend to monopolise the whole sound device.
Meanwhile, Linux became more and more 64 bit, but the flash plugin didn't. So you either had to run a 32-bit browser (because plugins were a library the browser loaded), or use some crazy proxy plugin thing (nspluginwrapper[1]) that someone made that was 64 bit at one end and 32 bit at the other.
Also meanwhile, other platforms got Flash player 6, and 7, and beyond, and still Linux was stuck on 5 or whatever. Then Mike Melanson[0] came along and spent some serious time and energy re-porting Flash player to Linux and made a lot of people very happy because now we had AV sync and no 32-64 bit bodges and fewer and fewer crashes and so on.
After a few years(?) he stopped, not sure why, and things slowed down. But by then it felt like flash was starting its decline, and what we had worked well enough, and Adobe seemed to be keeping basic updates happening anyway.
Now more and more I forget about installing `flashplugin-installer` or whatever Ubuntu calls it these days and don't notice that it's not there.
Note, aside from Mike's name, this is totally from memory so significant details may be wrong. Go read Mike's archived blog, it'll probably give you a much more detailed insight. Might do that myself.
I don't know, checked more - 1999 Macromedia introduces Flash Player for Linux [1], I see traces of v5, v6, v7 [2], [3]. Never heard about them, in my circle there was consensus "flash does not work on Linux". Strange
So technical availability was issue just in 2006-2007. Yes, I remembered flash was ads so without plugin it was ads free experience before AdBlock. I liked it a lot. The only issue was "copy to clipboard" buttons, but could work around.
[1] https://en.wikipedia.org/wiki/Gnash_(software)
[2] http://web.archive.org/web/20061030024511/https://www.howtof...
[3] http://web.archive.org/web/20080430162104/https://www.howtof...
[4] https://web.archive.org/web/20110511095927/http://blogs.adob...