I’d had a set of irons for about 25 years before I got custom fitted for some irons about 3 years ago, indoors using GC quad - it was the first time I’d ever been fitted for clubs and I was sceptical so didn’t end up buying the clubs.
A month or so later, I went to a different pro for a fitting outdoors on the range and they identified the same swing spec, so I felt a little more convinced by the process (and it made more of a difference to be able properly see the ball flight and carry).
I love my clubs and have a lot of confidence in them (much more than my old set).
However, over the past year or so, I think my swing has changed significantly (I now go left instead of right) so am probably due a refitting!