What I Am Reading:
At first, I really didn’t get this at all. I read that it was impossible, but I didn’t understand how.
As I read the mailing list, I got progressively more confused. I really didn’t understand what was being talked about – isStandard, Anyone-can-spend scripts, etc.
But finally, I came across the quote below, and it illuminated it all for me very well.
Since a soft-fork is a restriction of the consensus rules, I think the only way to have an un-soft-forkable cryptocurrency is creating a cryptocurrency where no transaction is valid.
Imagine I build a very minimal cryptocurrency where in the transaction output you only indicate the public key to send your coins to and the amount. One can still soft-fork it by deciding that, from now on, only even amounts are valid or only public keys that are a multiple of 10 are valid.
After I read this I understood the context of all the other elements they were discussing on the mailing list.