Unfortunately it appears that the packagers have disabled SBSMS, which is the time stretch library used by Sliding Stretch.
This also means that the high quality settings in “Change Pitch” and “Change Tempo” are also unavailable.
I don’t know why they have done that.
Probably the best option if you want this feature, is to build Audacity from the source code.
I posted instructions for Ubuntu 20.04 here: https://forum.audacityteam.org/t/building-audacity-2-4-2-on-ubuntu-20-04/57719/1
I think those instructions should also work on the latest Mint, but I have not tested.