We generate a daily changing identifier using the visitor’s IP address and User Agent. To anonymize these datapoints, we run them through a hash function with a rotating salt.

hash(daily_salt + website_domain + ip_address + user_agent)

This generates a random string of letters and numbers that is used to calculate unique visitor numbers for the day. Old salts are deleted to avoid the possibility of linking visitor information from one day to the next.

Full details are here: https://plausible.io/data-policy

It depends on whether they retain or can reproduce the salt for a given date.

The rule in effect is- a person knows the IP their ISP granted them on the dates they were granted. They ask- do you have any records of me from these IPs on these dates.

Assuming Plausible keeps the record of salt by date, the answer is yes, we have records of you, because they can retrieve the salt, recreate the ID, and locate the records.

If they do not retain the salt, in contrast, they cannot respond to individual requests for their records and that would also imply they are not able to do day over day returning visitor calculations.

Old salts are deleted to avoid the possibility of linking visitor information from one day to the next. So yes, there's no way for us to know whether the same person returns to a website on another day. See https://plausible.io/data-policy

That is deterministic, but the key thing is that it is not reversible

Technically, you could enumerate all four billion IP addresses (multiplied by all common user agents) to reverse it. This is, however, prohibitively expensive for tracking, so I think it does the job.

Not without the salt, which they delete every day. Pretty much impossible.

Is the salt key stored, or is it discarded?

Old salts are deleted to avoid the possibility of linking visitor information from one day to the next. See https://plausible.io/data-policy