Seems like the volume release of the Model 3 has supported Model S prices.
While there may have been more Model S come on the market because some are trading in their original Model S for a new Model 3, there are also a lot more people coming into the Tesla market.
Many potential buyers of the Model 3 got tired of the long wait, and began to take a fresh look at the Model S and X. A used S or X was close in price to a new $50-60K Model S/X. For many the Model S was much better looking, proven, smoother and quieter. It was much better for families due to additional sitting options and the hatch back design allowed for superior carrying of stuff.
With constant OTA updates the older cars had many of the newer releases as the 3.
I have seen no dramatic drops in the prices of S/X, other that what you might expect as those cars get older and more miles.
Overall used Tesla seems to have a much lower depreciation than competitive ICE vehicles.
Important, when calculating % of depreciation you consider that most USA Tesla received around $10,000 in tax incentives when new. Ignoring that fact can make the cars seem to be depreciating faster than they really are.
As an example, my X received tons of tax benefits as I own a small/profitable business. The X stickered at $85,000, but the Section 179 write offs gave me a net cost of around $35,000. Could easily sell it for more than that now. Same with many of the Scandavanian Countries. They recieve so many purchase incentives that they can mostly buy a new Tesla and drive it for several years for free, especially if the incentives go away after they purchase.
Seems like every country has their own circumstances. It the cost of fuel is high in your region, but you can charge up on Solar, the economics quickly make buying a Tesla a no brainer.