Has IOTA foundation planned a system of private transactions (whatever the solution considered "zero proof" or other approaches) to support better the privacy ?
The IOTA Development Roadmap (March 2017) mentioned Private Transactions research, through coin mixing first and then through more extensive technologies such as zero knowledge proofs.
There was a paper about this topic released on October 2017: Improving the Anonymity of the IOTA Cryptocurrency.
An IOTA Mixer was released into the testnet 1 month later: IOTA Mixer v1 Released.
There has been no noteworthy information regarding this topic since the release of IOTA Mixer v1. In this blog post, What’s Next: Current IOTA R&D Projects (June 2018), Private Transaction was not mentioned. From what I can tell, Private Transaction research is currently not a priority for the IOTA Foundation.