I don't know about Umami, Plausible describes how they solved this here: https://plausible.io/data-policy, under the section "How we count unique users without cookies"
TL;DR: They derive an identifier from IP address and User Agent using an hash, allowing them to have a tracking identifier without storing Personal identifiers (the IP address)
TL;DR: They derive an identifier from IP address and User Agent using an hash, allowing them to have a tracking identifier without storing Personal identifiers (the IP address)