Yep, get the part about open vs closed systems.
The question is still: why does it seem like now (like the last 10 years?) there is more of a concern about thermal expansion?
I never heard anything about this 10 years ago. Was it happening but people didn't recognize it?
Or has something changed, like more check valves being used? Or water companies using higher pressures, leading to more of us needing PRVs?