Heretic of The Seventh Circle
"Adventurer" is just kind of a meta designation from the get go. They could be irregulars, or militia. Heck, there are plenty of games where the PCs are explicitly endorsed by legal authorities as members of broader organizations.
There are related questions we can ask too: are Witchers "adventurers" ? Was Steve Irwin an "adventurer" ? (Hell, was Steve Irwin a Witcher?)
By an definition related to dnd that I’ve ever seen outside of this thread, Witchers and Steve Irwin are definitely adventurers.
Using the extremely narrow “fame and fortune” definition, I may have to change my vote to a much rarer option.
Using any definition that includes most PCs I’ve ever seen, my answer is probably “not unheard of”, but only because there isn’t an option between that and “a dime a dozen”.