I could not easily find it on the website, but I remember reading about how they do it, basically the ID is generated by hashing the IP + user-agent + a salt key that is changing on a daily basis.
So, no, I do not think it is deterministic.
So, no, I do not think it is deterministic.
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
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.