Before an SSD sector1 was ever written to, it looks like all filled with zeros.
So, if I write all zeros to a sector, for the purpose of functionality, it will look just like a free one. Thus the controller has a technical possibility to treat it as such. My limited knowledge of IC architecture says that hardware-wise, slowdown from a circuit testing for all zeros would probably be negligible, if any at all.
The question is: does any flash/SSD controller actually implement this or anything similar?
It looks even more applicable to flash memory storage connected via interfaces that don't have the TRIM command, like USB.
In the answers posted so far, a few people outlined possible show-stopper issues. Yet they all turned out to be non-issues. Unless there's evidence those really are serious problems, please do not authoritatively claim they are but rather honestly say you're only hypothesizing that.
1A logical sector i.e. what the host sees.