We performed ab initio valence-space in-medium similarity renormalization group calculations based on chiral nucleon-nucleon and three-nucleon interactions to study the neutron-rich nuclei around $N=28$, where the experimental and theoretical evidence point to a rapid transition from spherical to rotational regimes. First, the rotational spectra of $^{40}$Mg, $^{42}$Si, and $^{44}$S are established theoretically, anchored by the calculated E2 transition strength. The results suggest that $^{40}$Mg and $^{42}$Si exhibit large deformations. After that, the erosion of the $N=28$ shell closure, especially the evolutions of shape deformation and the emergence of shape coexistence, are discussed in $N=28$ even-even isotones ranging from $^{40}$Mg to $^{48}$Ca. Finally, the shell evolution of $N=28$ in Mg and Si chains is discussed based on the experimental and predicted excitation energies of low-lying states. The results from our ab initio calculations align well with available experimental data, furnishing essential insights into the structure of neutron-rich nuclei, particularly deformations in the vicinity of $^{40}$Mg and $^{42}$Si.