  We present results for pseudo-critical temperatures of QCD chiral crossovers
at zero and non-zero values of baryon ($B$), strangeness ($S$), electric charge
($Q$), and isospin ($I$) chemical potentials $\mu_{X=B,Q,S,I}$. The results
were obtained using lattice QCD calculations carried out with two degenerate up
and down dynamical quarks and a dynamical strange quark, with quark masses
corresponding to physical values of pion and kaon masses in the continuum
limit. By parameterizing pseudo-critical temperatures as $ T_c(\mu_X) = T_c(0)
\left[ 1 -\kappa_2^{X}(\mu_{X}/T_c(0))^2 -\kappa_4^{X}(\mu_{X}/T_c(0))^4
\right] $, we determined $\kappa_2^X$ and $\kappa_4^X$ from Taylor expansions
of chiral observables in $\mu_X$. We obtained a precise result for
$T_c(0)=(156.5\pm1.5)\;\mathrm{MeV}$. For analogous thermal conditions at the
chemical freeze-out of relativistic heavy-ion collisions, i.e.,
$\mu_{S}(T,\mu_{B})$ and $\mu_{Q}(T,\mu_{B})$ fixed from strangeness-neutrality
and isospin-imbalance, we found $\kappa_2^B=0.012(4)$ and
$\kappa_4^B=0.000(4)$. For $\mu_{B}\lesssim300\;\mathrm{MeV}$, the chemical
freeze-out takes place in the vicinity of the QCD phase boundary, which
coincides with the lines of constant energy density of
$0.42(6)\;\mathrm{GeV/fm}^3$ and constant entropy density of
$3.7(5)\;\mathrm{fm}^{-3}$.
