My original knowledge of automotive stuff comes from what I saw my Father do, what my Father told me and more so from reading his about 1947 or so auto mechanic’s trade school book set (He never worked as a Mechanic).
Which is where my knowledge of Wax being used in the capsule on thermostats comes from but at the same time I cannot be sure what is used in more modern times. Functionally speaking I have never had the need to update my knowledge.

https://en.wikipedia.org/wiki/Wax_thermostatic_element

"The wax thermostatic element was invented in 1934 by Sergius Vernet (1899–1968).[1] Its principal application is in automotive thermostats used in the engine cooling system."
"Wax thermostatic elements transform heat energy into mechanical energy using the thermal expansion of waxes when they melt."
"This most common modern form of thermostat now uses a wax pellet inside a sealed chamber.[6] Rather than a liquid-vapour transition, these use a solid-liquid transition, which for waxes is accompanied by a large increase in volume. The wax is solid at low temperatures, and as the engine heats up, the wax melts and expands. The sealed chamber operates a rod which opens a valve when the operating temperature is exceeded. The operating temperature is fixed, but is determined by the specific composition of the wax, so thermostats of this type are available to maintain different temperatures, typically in the range of 70 to 90°C (160 to 200°F).[7] Modern engines run hot, that is, over 80 °C (180 °F), in order to run more efficiently and to reduce the emission of pollutants."

__________________
84 300D, 82 Volvo 244Gl Diesel

In 1947, automotive thermostats were still alcohol bellows powered. Wax thermostats didn't make it into civilian use until after the Korean War. If you want to know anything about thermostats:

https://www.coolcatcorp.com/thermostats/Thermostats.html

Stories of thermostats failing out of the box are usually just that. In writing my e-book, I tested hundreds of thermostats, never had one out of the box that simply failed to open. That said, there were a lot of calibration issues. Mercedes engines are especially sensitive to calibration, because the "cool side" thermostat design requires high hysteresis. If the wax is too fast or too slow, you'll see things on your gauge that you could never test in the pot.

As far as failing in service, a thermostat will last as long as the seal on the wax element lasts. The wax itself never can wear out or lose calibration. But once the wax leaks out or water leaks in, it's over. There are a lot of no-name thermostats on the market that have poor seal design and will fail early. And others with poor flow characteristics or bad calibration that will disappoint. You can't tell by observation which are good and which are bad, and as I mentioned, pretty much all of them will open and close in a pot of water right out of the box. There has also been a lot of consolidation in the business, so what you get is a bit of pot luck. At least one of the major brands is selling Made in China devices in Made in Germany boxes. When I had the Benz, I used Behr exclusively, but I can't speak to current products.

Thank you for that!

__________________
1981 240D 4sp manual. Ivory White.