To me, most "pilot damage" is either feet (cockpit eggshells) or vertical loads that damage/break the spine.....assuming not a major hit.
Taken from someone that has "broken their back, twice", but not in a sailplane.
Only looking at accidents I have read or been an early party to recovery.

To me, airbags (unless maybe under the pilot) are a ton of cost with little to no benefit, just my opinion......

"If airbags were so great, why don't ANY auto motorsports require them?".
Because, a good set of well anchored belts will be about the best bet.
Aircraft crashes tend to have a vertical motion, belts don't help there. Crush structure does.......most wouldn't put an airbag under your back/butt where most damage is done.
Cockpit structure for your feet/lower legs HAS been done.
Ask Andy Gough.......I helped first responders during his incident many years ago in PA.
Glad to see him return to contests.