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
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.