You are right.
I installed win-acme.v2.2.9.1701.x64.pluggable
The executable file is wacs.exe
The reason why I thought it is more related to LetsEncrypt than just a third party client is that the emails about renewals come from what it says "Let's Encrypt Expiry Bot", with an email "expiry@letsencrypt.org"
So where does this client get the list of domains to renew the certificates? From my server or from the LetsEncrypt server(s)? And how do it delete one from that list, wherever it is maintained?
Also, do you think that this is the most common and widely used client for IIS on Windows servers?
What are other reliable and well-established clients? Would you suggest some better ones or the best one in your opinion?