Many DC-DCs and PWM controllers have this feature.
It's actually abnormal, to not - it's a common problem
(not everybody can go slug the upstream source full-on
or something bad's gonna happen).
If you know Cload and Iload, and max allocated Iin, you
can figure the Iin allocatable to filter charging (Cload)
and hance dV/dt, dV is your output voltage so dT is
your soft start ramp time.
Now operating a converter below its undervoltage lockout,
that's not gonna work.
You might not want to bother with trying to cover the
whole range of output voltages. Look to your "use model"
for insolation and cell I, V variation. You might be willing
to just forget about the last (say) 5% of delivered energy
per day if it doubles material cost and triples design effort.
Just because there is some condition where MPP is 1.8V
doesn't mean anything in itself; maybe that's a case where
there's no meaningful power output anyway, so forget it.
But if your source is solely solar cells I don't think you
need soft start, because you want to take every bit of
their current and pump it out.
You might want to think a bit about architecture.
You might do better with a very crude boost at the
front to pull out all the energy you can to a working
semi-regulated voltage, and then a buck charge
controller to make the battery happy.
Further if you are unable to use all the panel output
and know this for sure, you might scrub off the high
end by cell shunting, pretty cheaply. Better to return
the current intra-cell at low voltage and power, than
send it out to stress other things and make more heat
everywhere. A good enough shunt FET is dirt cheap
and one more pad on a chip, close enough to free.