Afaik sub satoshi doesn't exist in the blockchain, and less than 1 sat/vbyte transactions do not get broadcasted
Correct
It would take big code changes to make subsatoshi happen, isnt it?
I don't know, but AFAIK it requires hard fork.
But 1 sat/vkb could be a nice solution and maybe not that hard to implement.
It's very easy, you just need to set
minrelayfee on
bitcoin.conf or convince Bitcoin Core (and other full node software) developer to change the default relay fee value.