The meaning of "all but X"
I've heard conflicting accounts on what exactly "all but X", as in "our history has been all but forgotten" is supposed to mean.
Some people I've asked say it means exactly that, all BUT X, as in everything EXCEPT X. So if something is all but forgotten, it's everything EXCEPT forgotten. Others have said it simply serves as emphasis, in this case meaning something has most definitely been forgotten.

So my question is: does it mean something has NOT been X, or has CERTAINLY been X?

Question

The meaning of "all but X"
I've heard conflicting accounts on what exactly "all but X", as in "our history has been all but forgotten" is supposed to mean.
Some people I've asked say it means exactly that, all BUT X, as in everything EXCEPT X. So if something is all but forgotten, it's everything EXCEPT forgotten. Others have said it simply serves as emphasis, in this case meaning something has most definitely been forgotten.

So my question is: does it mean something has NOT been X, or has CERTAINLY been X?

Answer

You can consider it like this: On a scale of 'forgottenness' from 0 (remembered) to 100 (forgotten) 'all but forgotten' is 99. It's not quite forgotten but very close.
That's the idea. The person is not willing to say 'it is forgotten', but they are saying that it almost is, and/or will be soon.

Answer

"all but forgotten" would me the same as "almost forgotten", but "all but x" can mean "all except x" too.

For example, "all but 3 of the people I invited came to the party"  would mean that everyone invited showed up to the party except for 3 people.