@CFS_for_19_years , sorry I did not respond directly earlier.
I have had a few neuro-cognitive evaluations over the last several years. They do usually offer up an estimate of pre-morbid FSIQ's, typically with a 90% confidence level. If I remember my stats correctly, a 90% confidence level is pretty poor. Naturally, they also take a stab at your current IQ.
But these evaluations are much more than just IQs. Processing speed, judgement, visual memory - there are several cognitive domains gauged.
Maybe you are correct when you say that IQ's may not diminish. I agree with Bob that some of these tests may be looking at the wrong things; or rather, there may be better vehicles to capture our deficiencies than the normal battery of cognitive tests provide.
I do not put much faith in the usual neuro evaluations because I know they got some of my values wrong, including my IQ. But the rank and file of the medical community, and our entire culture by and large, do heed them.
We all know that "brain fog" is inadequate. Unfortunately, how we can demonstrate our deficits is not always easy.
So if normal cognitive testing yields "All is normal" (and in your case, superior), we know it is not, at least not for us. Maybe with these tests that
@Bob and
@mango have identified, our chances at more representative results will improve.