We present a detailed spectral and timing analysis of Cygnus X-1 with multi-epoch observations, during $2016$ to $2019$, by SXT and LAXPC on-board AstroSat. We model the spectra in broad energy range of $0.5!-!70.0,rm{keV}$ to study the evolution of spectral properties while Cygnus X-1 transited from hard state to an extreme soft state via intermediate states in 2017. Simultaneous timing features are also examined by modelling the power density spectra in $3.0!-!50.0,rm{keV}$ . We find that during high-soft state observations, made by AstroSat on Oct $24,,2017$ (MJD $58050$), the energy spectrum of the source exhibits an inner disk temperature (kT$rm_{in}$) of $0.46!pm!0.01,rm{keV}$ , a very steep photon index ($Gamma$) of $3.15!pm!0.03$ along with a fractional disk flux contribution of $sim!45%$. The power density spectrum in the range of $0.006!-!50.0,rm{Hz}$ is also very steep with a power-law index of $1.12!pm!0.04$ along with a high RMS value of $sim!25%$. Comparing the spectral softness of high-soft state with those of previously reported, we confirm that {it AstroSat} observed Cygnus X-1 in the `softest state. The lowest MAXI spectral hardness ratio of $sim!0.229$ corroborates the softest nature of the source. Moreover, we estimate the spin of the black hole by continuum-fitting method, which indicates that Cygnus X-1 is a maximally rotating `hole. Further, Monte Carlo (MC) simulations are performed to estimate the uncertainty in spin parameter, which is constrained as a$_{ast}>0.9981$ with $3sigma$ confidence interval. Finally, we discuss the implications of our findings.
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