I'll present this as
preliminary data to get the conversation going:
I looked at the amount of turnover in the NHL, starting with the 1949-50 season. I compared the rosters in the current and following season only (ie 1949-50 to 1950-51). I arbitrarily decided that a player needs to play in at least 10 games in both years. This is for skaters only (no goalies).
This was a difficult report to generate and it's likely there's random errors in the data. Without doing a deep dive, I'm not sure how much this affects the results.
With these disclaimers in mind - the long-term average is 81% (which means there's 19% turnover).
There was only a single season with less than 10% turnover. That was 1966-67. That's not a surprise - the NHL doubled in size the next league. Virtually everyone who was good enough to play in the six team league was able to get a spot the next year.
The two seasons with the most turnover (right around 25%) are 1951 (no idea why) and 2004 (lots of older stars from the previous generation couldn't withstand a year away from the rink in their 40's and/or weren't fast enough to keep up after the lockout when speed was emphasized - Lemieux, Messier, Yzerman, Hull, Francis, Robtaille, Oates, Mogilny, Andrehychuk, Damphousse, MacInnis, Stevens, Leetch, etc).