  The COMPASS Collaboration performed measurements of the Drell-Yan process in
2015 and 2018 using a 190 GeV/c $\pi^{-}$ beam impinging on a transversely
polarised ammonia target. Combining the data of both years, we present final
results on the amplitudes of the five azimuthal modulations in the dimuon
production cross section. Three of these transverse-spin-dependent azimuthal
asymmetries (TSAs) probe the nucleon leading-twist Sivers, transversity, and
pretzelosity transverse-momentum dependent (TMD) parton distribution functions
(PDFs). The other two are induced by subleading effects. These TSAs provide
unique new inputs for the study of the nucleon TMD PDFs and their universality
properties. In particular, the Sivers TSA observed in this measurement is
consistent with the fundamental QCD prediction of a sign change of naive
time-reversal-odd TMD PDFs when comparing the Drell-Yan process with
semi-inclusive measurements of deep inelastic scattering. Also, within the
context of model predictions, the observed transversity TSA is consistent with
the expectation of a sign change for the Boer-Mulders function.
