where is subiquity's snap gone?
Solution 1:
I'm answering my own question as it was a silly one after all.
snap download doesn't mean snap install : the first fetches the snap package and the other effectively install it on the system.
So the missing part was
snap ack subiquity_2584.assert
snap install subiquity_2584.snap
But not a good idea to install subiquity on a Desktop as advertised by the warning. Better do that on a VM.