The time-dependent $CP$ asymmetries of $B0\to \pi +\pi -$ and $Bs0\to K+K-$ decays are measured using a data sample of $pp$ collisions corresponding to an integrated luminosity of 1.9 fb$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$, collected with the LHCb detector at a centre-of-mass energy of 13 TeV. The results are \begin{eqnarray*} C_{\pi\pi} & = & -0.311 \pm 0.045 \pm 0.015, \ S_{\pi\pi} & = & -0.706 \pm 0.042 \pm 0.013, \ C_{KK} & = & \phantom{-}0.164 \pm 0.034 \pm 0.014, \ S_{KK} & = & \phantom{-}0.123 \pm 0.034 \pm 0.015, \ \cal {A}^{\mathrm{\Delta\Gamma}}{\mathit{KK}} & = & -0.83\phantom{0} \pm 0.05\phantom{0} \pm 0.09, \end{eqnarray*} where the first uncertainties are statistical and the second systematic. The same data sample is used to measure the time-integrated $CP$ asymmetries of $B0\to K+\pi -$ and $Bs0\to K-\pi +$ decays and the results are \begin{eqnarray*} A{CP}^{B^0} & = & -0.0824 \pm 0.0033 \pm 0.0033, \ A_{CP}^{B^0_\mathrm{s}} & = & \phantom{-}0.236\phantom{0} \pm 0.013\phantom{0} \pm 0.011. \end{eqnarray*} All results are consistent with earlier measurements. A combination of LHCb measurements provides the first observation of time-dependent $CP$ violation in $Bs0$ decays.