A related and very critical point that I've not seen mentioned or
written anywhere about higher altitude soaring (17K and above) is that
it is much easier to exceed Vne because of human factors combined with
thinner air.
1) Less noise: Because the air is thinner, there is less air flow
noise in the cockpit. Many pilots use air flow noise as a secondary
way to monitor airspeed.
2) Lower indicated Vne: If your Vne is say, 145 knots, then at 18K,
your indicated Vne limit is 106 knots.
3) Lower pitch angle: Your nose and control stick do not need to be
pointed down as much to get to 106 knots compared to 145 knots. Like
air flow noise, the amount you push the stick forward is less to get
to Vne.

Combine #1 and #3 and a few moments inattention to the airspeed
indicator, and you can quickly exceed indicated Vne. This is a key
risk in cross country wave flying, where, at least out west, you can
run at 17.5K at Vne for long stretches. The other case is when, again
out west, you leave the thermal at 17.5K, but then run into more lift
as you accelerate and think, "I'll run it up to Vne" to avoid busting
18K and not monitor the airspeed indicator.

Now consider if you are at 24K, not much higher than 18K, either
because you have a wave window or clearance, and now indicated Vne is
98 knots.

Just some related points to keep in mind....

Kemp