The closest that I can find is section 15 of the Citizenship Act -
https://laws-lois.justice.gc.ca/eng/acts/C-29/section-15.html - which basically just says that we have an obligation to answer everything truthfully. I understand this to mean no misrepresentation.
I'm not really worried whether anyone wants to dsclose in more detail than I think is necessary. It is just my opinion.
But as for the misrepresentation aspect: there is a basic background concept here of
materiality. I'm not going to try to write an academic law textbook chapter on this, so let's just simplify and say it has to be
relevant to the inquiries they're making, and
important enough that it might conceivably change the way someone evaluating the file is going to approach it (not just what the final decision is but the way they evaluate the file and the depth of work they may put into evaluating it). [I suspect there may also have to be some determination of
intent but that alone is a complex topic.]
And this is where it gets into subjectivity and the judgment of the applicant. We can all imagine occasional work or activities that almost anyone looking at the file would say is not worthy of disclosure - someone giving occasional tennis lessons, getting small fees for giving a lecture, your part-time clown show for disadvantaged kids, etc. Personally I think most part-time work falls into this, and indeed most ... what I'll call non-principal activity. The main purpose of providing this info is met, and there are few (IMO) plausible reasons to think the info about other activities is material in the context of the citizenship app.
That said: there are some that might be relevant and need to be disclosed. See my (somewhat) facetious hypothetical of part-time employment for a foreign intelligence agency. Hide that and they might use the misrepresentation angle.
Being in the reserve of the Canadian Forces might be worth disclosing, esp if there have been any deployments - not that 'hiding' it would likely be an issue, on the contrary, would let IRCC check some things about presence in Canada. (Yes, there might be cases where it's to the applicant's advantage to provide some of this 'extra' info).
Where does someone's specific example fall? As indicated, I don't think most need to be disclosed, but applicants can decide it any way they wish. In many other contexts I recommend maximum disclosure (just to avoid potential misrepresentation). In this case, I don't - and I think a lot of the 'noise' about this in various internet forums and chat groups etc are getting people wound up about (mostly) nothing. I think it's mostly exaggerated.
But let each applicant use their own judgment. And hopefully stress about it less. If providing what I think is 'too much' info makes you feel better - find, no skin off my nose.