The trouble is, there was a lag between medical knowledge of concussion changing, and the game reacting to that change, and that lag is where any law suit is going to target.
Bringing up anything before 2005 (CTE first identified) isn't remotely useful, as the longer term effects simply weren't known. You also need to allow for a lag between experts in the field being aware, and rugby administrators being aware.
I certainly remember 2008 when Justin Harrison was KOd a couple of times in a match, and players from both teams ran obstruction to stop the medical staff getting to him early, I was a lone voice criticising the behaviour, whilst most were lauding it - and I wasn't criticising because of CTE (I didn't know it existed at that point), but second impact syndrome (which has long been known and acknowledged, and comes down to "players (should) know the risk").
By memory, it wasn't until 2013 with the film "concussion", and 2014 with George North's incidents that it became a widely known concern, and rugby started to react (ETA, it was a little before that, just as trials or 100% adopting medical concensus recommendations).
RFU introduce concussion education courses in 2014, HIAs introduced in 2015 (first trialled in 2012). There was also a change in 2012, as a knee-jerk before the film was released, that didn't really understand the problem, the one that had Barry O'Driscoll resigning - but was still ahead of rugby's peers.
So you've really got that 7 year period when CTE was known, but the rugby authorities didn't know.
The key time (for the authorities) is going to be between whenever it was that Barry O'Driscoll started complaining about it (I don't know when that was, and it being taken more seriously in 2014/15.
After that, you've either got to claim the the reaction wasn't strong enough, which would be a tough claim to make stand, as medical knowledge was changing year by year, as was rugby's reaction - or the claim is now against individual medics / coaches for ignoring the regulations.
It's not like the NFL example when the authorities were explicitly briefed, and given the opportunity to be involved from the very start, and actively covered it up.
ETA, in trying to confirm dates for things, especially Barry O'Driscoll's unhappiness - there was also an general medical panel convened in 2004 to look at it, with the IRB enacting the recommendations (3 weeks off, or return to play with a neurologist signing off).
So as of 2004 rugby per se is in the clear for acting in accordance with best medical knowledge of the time.
That seems to have been the case in 2008 as well - though the
article I've found skips over that one, Dr O'Driscoll doesn't seem unhappy about it yet.
It was the IRB's reaction to the 2012 4-yearly concussion summit that upset Dr O'Driscoll - basically, the first attempts by the IRB to take more serious action were a bit bumbling, until 2014...
So really, and law suit has a 2 year window to claim that the sport's authorities weren't following best advice. After that, advice was being followed, and getting more stringent with each trial and new enactment.
After that, it's down to, for example, the Welsh medical staff being the only people watching the match to not notice that North was out cold after a knee to the head.
NB from that article Re: 2012 "Support for his convictions were hard to come by on the medical front, but within the rugby community, O'Driscoll was applauded by many of his peers." So even in 2012 it looks like the IRB was going with medical consensus, but Dr O'Driscoll wanted more.
It's going to be a tough law suit that claims "So you were following the medical consensus recommendations of the time, enacting the recommendations of the experts in the field - we feel that this was incompetent"
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