  Femtoscopy studies of pion radiation in heavy-ion collisions have been
conducted extensively at all available collider energies, both theoretically
and experimentally. In all these studies a special interest is given to $m_{T}$
dependency of pion femtoscopy radii, usually approximated by a power-law
function at transverse momenta above 200 MeV/$c$. However, the radii behaviour
has been much less explored for the ultra-soft pions, possessing the transverse
momentum comparable to or lower than the pion mass. For many experimental
setups this region is difficult to measure. In this work we present theoretical
calculations of pion emission in the ultra-soft region in the two hybrid models
-- iHKM and LHYQUID+THERMINATOR2. Along with the particle transverse momentum
spectra, we present the calculated femtoscopy radii, both in one-dimensional
and three-dimensional representations. We investigate the radii dependence on
pair $m_{T}$ and observe, in particular, a departure from the power-law
behaviour at ultra-soft momenta, potentially reflecting a decoupling of such
slow pions from the rest of collectively expanding system. We provide the
theoretical interpretation of this result and discuss its significance, in
particular, for the ongoing non-identical particle femtoscopy analysis for
pairs consisting of a pion and a baryon (or of a pion and a charmed meson).
