  The surprising discovery of tripling the superconducting critical temperature
of KFe$_2$As$_2$ at high pressures issued an intriguing question of how the
superconductivity in the collapsed tetragonal phase differs from that in the
non-collapsed phases of Fe-based superconductors. Here we report $^{89}$Y
nuclear magnetic resonance study of YFe$_2$Ge$_{x}$Si$_{2-x}$ compounds whose
electronic structure is similar to that of iron-pnictide collapsed tetragonal
phases already at ambient pressure. Fe(Ge,Si) layers show strong ferromagnetic
spin fluctuations whereas layers are coupled antiferromagnetically -- both
positioning the studied family close to a quantum critical point. Next,
localized moments attributed either to Fe interstitial or antisite defects may
account for magnetic impurity pair-breaking effects thus explaining the
substantial variation of superconductivity among different YFe$_2$Ge$_2$
samples.
