By its very nature, an analytics product must process personal data.

Personal data is "any information relating to an identifiable individual" (see GDPR art 4(1).

Your IP address, browser and OS (via user agent), the website you visited, the pages you visited, time of visit, the site you came from (via referrer) are all personal data.

If Plausible have put a US owned cloud provider in-front of their Hetzner infrastructure, even if for a legitimate purpose (CDN, DDoS prevention) then that is likely an unlawful transfer of personal data to the USA.