This particular failure mode has nothing to do with the year, it spans 2005 up to at least 2014 for confirmed cases.
The failure mode is wear to that exact circlip during downshifts. It dislodges up onto the shaft and then at some point the gear walks and boom during a 3 to 4 upshift.
The cause is very simple, it's one of these three or a combination of them:
A) Circlip installed backwards
B) People kicking the lever downshifting or stomping on the heel plate if using a heel shifter.
C) Bottom of Groove OD at upper limit or slightly over tolerance and shaft OD being at lower limit or slightly under limit.
This IS actually a reasonable class action or at least a NTSB forced recall issue. It can definitely lead to accident/injury. I was leaned over near the edge of the tire, full throttle going 3 to 4 at 7500 RPM or so when I did it while passing traffic.
Triumph is WELL aware of the issue.