No it's not available and won't be available going forward (unless Microsoft changes it's strategy). You can still install the official 10586.164 build from Windows Insider but it will be unsupported. Performance is more or less acceptable and stable enough for daily driver but you might want to read some forum posts to identify any show stopping bugs before you decide to upgrade.
If you use Insider on the Release Ring. The same version is fetched as the recent public Upgrade received by these phones (10.0.10586.164): Phones Eligible for Windows 10 Mobile Official.
If your phone is not on the supported list, then 10586.164 (Threshold Final) is the end of the line. You may as well use Insider and select Production to un-enroll your phone from Insider, but at least you have W10M.
If your phone is on the eligible list and you go Insider and stay on the Release ring you get the next public version early as Release ring in effect is the RTM.
I have upgraded to build 218 with no problem. My experience with my 820 (1Gb) is great. Have not had any problems and it seems just as fast as 8.1 to me. They really should have another look at this for the 820.
Hi Klyt , how's the update on your 820 ? Are you experiencing some troubles with that ? My Lumia is also 820 and is currently Denim, I want to upgrade it also to win10 and I need some help