History of Apple Inc.: Did Steve Jobs use his own iPhone in his keynote demos?

There was a lot of preparation and safeguards involved in any Steve demo.  The slides were edited and refined over several weeks and then every aspect of the presentation was practiced and rehearsed over and over in Town Hall.  Steve would have multiple demo devices and for any demo involving OSX, there would be redundant machines that could be switched over if the demo machine took a dump.  Switch-overs were needed on several occasions and we would always breath a sigh of relief when the keynote ended.

Almost certainly not. Early iPhones had no screen mirroring capability, so they had to use modified iPhones that had a video output wired off the display controller (this is seen as a thick black cable in the keynotes). And of course, they always have redundant demo machines. iPhone models that support AirPlay Mirroring could, theoretically, be used unmodified for presentations. But given how flaky Wi-Fi can be at conventions, I doubt this was ever even seriously considered. I speculate that modern demos either continue to use modified iPhones, or use the HDMI output via Lightning, but with the software modified to do screen mirroring to HDMI.

To eliminate the risks that the messages do not get through, every good company cares a lot about the presentations they make. They have backup procedures, are doing rehearsals, are developing, changing the presentations. Would you take the risk to use your own phone, or just one phone, or a phone on which you could receive an unknown phone call during your vital presentation? Probably not.