This reminds me of when my grandpa told me the clicking we hear when we turn on a turn signal is from the old days when a cover used to visibly/audible "click"/close over the light, mimicking a blinking effect.
There were other turn signal approaches used before then, though not all cars had turn signals. Some of those approaches were mechanical, but still don't really align to your grandpa's claim.
https://www.qualityplusautomotive.com/blog/2020/september/th...
https://www.cartalk.com/blogs/jim-motavalli/strange-true-his...
https://www.youtube.com/watch?v=2z5A-COlDPk
Demonstrates thermal-based flashers as well as capacitor-based flashers. They are clicking because metal is moving and hitting metal.
Once upon a time, an "accidental" feature was that the thermal bimetallic flasher modules would flash much faster if the load was lower. So if you turn indicator flashed fast, you knew that one of the bulbs was out. I've seen this fairly recently; is it "designed in" to modern turn signal systems even though the bimetallic flasher units are long gone? And is that why there's still an incandescent bulb back there - because it's the easiest to monitor for not being functional by just watching the current draw?
Also once upon a time, you could pull out the thermal bimetallic flasher unit and replace it with an "electronic" one consisting of a transistor, a couple of passives, and a relay. Those made a very satisfying loud "CLICK CLACK" kind of noise that was impossible to miss. In my modern, car, I'm pretty sure the tick tock sound is synthetic, but it's also quiet enough that, at least at my age, it's easy to miss.