How common was it for founders to cash out on common shares in the Series C-E rounds the last decade?
Very. It's usually not this egregious though. Founders might want stable finances for their family or the ability to buy a reasonable home, but VCs want them to keep going, so VCs allow founders to cash out some of their equity so that founders don't go for an exit for the sake of their personal finances.
For the others involved from investors to employees it's never good to be working with someone who could (theoretically) stand up from a meeting and walk out to go on and do whatever they want forever.