Do undertaking obligations still apply if the sponsored relative never actually uses social assistance?

If a sponsored relative never actually receives government social assistance, there is generally nothing for the government to recover from the sponsor, since the repayment side of the undertaking is tied specifically to assistance that was actually paid. In that sense, if no assistance is ever provided, that particular financial exposure never comes up in practice.

That doesn't mean the undertaking simply doesn't apply, though. The undertaking is a broader legal commitment covering the full support obligation set out when it was signed, and it exists in full for its entire length — running from the day the sponsored person becomes a permanent resident — regardless of whether assistance is ever actually paid. The support obligation and the legal commitment behind it remain in place the whole time, even in years where nothing is ever claimed.

So it's more accurate to say the practical financial risk depends on whether assistance is paid, while the underlying legal obligation itself doesn't disappear just because no claim has come up yet. If you have questions about what your undertaking actually requires of you day to day, regardless of whether any assistance has been paid, it's worth speaking with an immigration lawyer to fully understand your ongoing commitment.