As said upthread, I suspect the only reason supermarkets will be putting up notices or making announcements is to be seen to be doing something to avoid grief on social media or flack from the unions - that's what happened after 19 July. It will however be entirely non-confrontational and if you don't wear a mask, which there is no compulsion to do from tomorrow, no-one will question you over it.
If it's anything like July, then around 80% of people may be wearing masks tomorrow, but that number will quickly drop rapidly.