I've heard that because rap in its earliest days was really improvisational, some early rappers explicitly didn't want their music getting recorded. To what extent is that true, and do we know about any early rappers and who were popular on the improv scene in their time but we don't have many recordings of?
Hi - I've only just seen this as I was looking through your user profile for another reason! If you haven't already seen it, I answered a similar question here which explains the difference between 'hip-hop' and 'rap', and what pre-'Rapper's Delight' rap was like.
And yes, there's plenty of early hip-hop acts whose success in the New York live scene didn't translate particularly well to mainstream recording success - chief amongst these is DJ Kool Herc, who has as much claim to inventing hip-hop as anyone else, and who didn't appear on an official record release until 1994 (when he appeared on a track by Public Enemy producer Terminator X). There's also the two crews featured in the 1982 movie Wild Style, the Cold Crush Brothers and the Fab(ulous) 5, who were both reasonably prominent in the New York scene in the late 1970s, and both of which are more famous for being involved in other people's big hits than for their own recordings. Famously, the rap in Blondie's 'Rapture' mentioned a member of the Fab 5, Fab Five Freddy (who's also in the video), while Grandmaster Caz of the Cold Crush Brothers allowed Big Bank Hank of the Sugar Hill Gang to use some of his lyrics on 'Rapper's Delight'.
I don't mean to discourage discussion here, and I'm not sure that this comment is ok under the rules(very sorry if so!) but I would also recommend posting this question on r/hiphopheads as there are many older people there who really know their HH history, just in case there somehow isn't anyone here who has that specialty!