Will "soft nucleophiles" undergo 1,4-conjugate addition even if the β-carbon is sterically hindered?
Your line of thought possibly is influenced by the Walden inversion which is characteristic for the nucleophilic substitution mechanism of $\mathrm{S_N2}$ where steric hindrance affects the rate when this umbrella-like structure flips upon entry of the nuclophile and departure of the nucleofuge around a formally $\mathrm{sp^3}$ hybridized center:

(credit)
The reaction shown by you however is an addition on to a carbon with molecular orbitals formally hybridized as $\mathrm{sp^2}$.  Similar to the Bürgi–Dunitz trajectory for the entry of a nucleophile on to a $\ce{C=O}$ bond a) it is plausible that the angle of attack for the 1,4-addition b) equally is tilted, rather than an attack from the back.

(composed with a picture from here)