You can edit on the front end and it will change, after the seven day pay-out, but if you look at a block explorer it will record this as a 'new post', the old original version will stay the same, in whatever block it was recorded - here's the original version of the Justin Sun fucky post on the block explorer - the post has been changed fundamentally on the front ends, but the original remains here, forever.

https://hiveblocks.com/steem/@justinsunsteemit/justinsunsteemit-witness-voting-policy

I assume a consensus hard-fork could remove some blocks (I think, I'm not sure), but failing that, we have immutability in the sense above.