When I asked Jeph in some Q&A about whether robots feel pain, he said something very close to "No, they're computers", and definitely used the word "computers".
I forget when this was, and it matters, since Jeph has steadily upgraded the status of the synthetics in the strip. It might no longer be the appropriate term. I vaguely remember it was after Momo's upgrade but can't even be sure of that.
For that matter, even without Jeph upgrading AIs' capabilities and rights over time (which, I do feel like the differences between Pintsize in QC #1 and the AIs in QC today can't
just be explained by in-universe evolution, there's definitely some retconning going on to flesh out the AI backstory)... pain response is something that could very easily evolve over time.
Pain response is something that evolves as a protective strategy, and AIs have some advantages over humans in this regard (there's evidence in-universe that they can evolve their neural nets
significantly faster than humans can).
I could actually believe that pain response wasn't known to exist in AIs even in a version of QC #1 that carries all of the retconning of the current universe, although I doubt it (considering
this strip, drawn after the AI civil rights retcon and including its effects, but set before QC #1).
Now, though? I actually can't believe that pain response
doesn't exist in the QC universe today, and we've actually seen proof that it can exist, right
here. Sure, that arc isn't depicting a
pain response, but it's depicting all of the senses and responses necessary for
quite vivid pain responses. Roko may well be an outlier, but all it takes is one outlier to disprove a negative.