Brownian endpoint sufficiency for a constant drift

For $Y_s=\lambda s+W_s$ and fixed $t>0$, subtracting $(s/t)Y_t$ leaves the Brownian bridge $W_s-(s/t)W_t$. It is independent of $(\lambda ,Y_t)$, since it is independent of $\lambda$ and has zero covariance with $W_t$ in their jointly Gaussian law. The observed path is determined by this bridge and its endpoint, so the bridge gives no further information about $\lambda$. Hence conditioning on the full observation filtration at $t$ gives the same posterior as conditioning on $Y_t$ alone.