It's not hugely common for me but also I've made it clear on multiple occasions that I don't want people to do that. When people do try that in real life, I typically shut them down quickly. In email, I immediately delete it without looking at any proposal and don't respond to it at all.
How often do you generally have random people pitch you stuff like that (not including obvious scam emails)?
I know unsolicited material is a big no-no for professional creative types. Just curious how common it actually is.