May 2017 to April 2018 down $15,000 per tracked auction results. This is from a subscription-only provider of high end car auction data.
The ship was sailing in late 2015 and by 2016 it was sinking. Recently a '77 quit at $100,000 on BAT. The '65 911 feeding frenzy prices are down as well.
The Turbo was not a limited production car and there still plenty to go around. Only the best will ring the bell. The also-ran auctions found on the fringes of most majors are where the "of questionable provenance" cars end up. I saw a lot of "iffy" cars at Amelia Island this year at these places. Some touted to have original paint where a friend who restores Porsches commented: "Looks like it was blocked with a brick".
No such thing as a permanent trend other than death and taxes so you may as well hold.
I still like the survivor market; the only-original-once cars.