I also don't think you're right that it never happened for the dedicated servers :) I'm only using Hetzner for dedicated servers, and found an email from 2022 where they mention price updates:
> Unfortunately, we are forced to increase the prices on these Server Auction models [...] old price 37.60 Euro -> 59.29 Euro, comes into effect 2022-03-03
Citing raising energy prices at that time.
Probably not for existing customers (their existing servers). I don't recall anything other than the IPv4 related increase in the past. I might be wrong of course as I've demonstrated already.