"Most web analytics tools do this by excluding certain IP addresses from being counted. However, we do not store the visitors’ IP addresses in our database for privacy reasons"
GDPR doesn't care about storage. Even if you just acquired personal information without processing it, you still had to be GDPR compliant.
In fact, the solution suggested above (only using a truncated IP address) would still require you to acquire and process the IP address and thus be subject to GDPR.
Their docs suggest as much https://docs.plausible.io/excluding/
"Most web analytics tools do this by excluding certain IP addresses from being counted. However, we do not store the visitors’ IP addresses in our database for privacy reasons"