> Google Analytics provides an IP anonymization feature.
The &aip=1 feature - in spite of it's name - does not provide any useful anonymity! As you can see in Google's own documentation (your 2nd link), when aip=1 GA claims that "the last octet of the user IP address is set to zero".
At best this can only group your IP with the neighboring 255 addresses. Google still logs the upper 24-bits of the address, which is probably enough to discover e.g. your ASN and geolocation. In practice, IP addresses usage is not perfectly uniform, so your actual "anonymity" is less than the theoretical maximum of 1-in-256. In general, the HTTP headers, cookies, etc will have at least 8 bits of unique entropy that more than makes up for losing the least interesting 8 bits of your IPv4 address.
This feature isn't designed to provide actual anonymity. The documentation even suggests the feature was designed to minimally satisfy certain legal or contractual obligations:
>> This feature is designed to help site owners comply with their own privacy policies or, in some countries, recommendations from local data protection authorities, which may prevent the storage of full IP address information.
Notice that this mentions pre-GDPR "recommendations" and that compliance is the goal, not user anonymity.
(side note: that documentation doesn't even acknowledge IPv6. Does the aip=1 feature even exist for IPv6?)
The &aip=1 feature - in spite of it's name - does not provide any useful anonymity! As you can see in Google's own documentation (your 2nd link), when aip=1 GA claims that "the last octet of the user IP address is set to zero".
At best this can only group your IP with the neighboring 255 addresses. Google still logs the upper 24-bits of the address, which is probably enough to discover e.g. your ASN and geolocation. In practice, IP addresses usage is not perfectly uniform, so your actual "anonymity" is less than the theoretical maximum of 1-in-256. In general, the HTTP headers, cookies, etc will have at least 8 bits of unique entropy that more than makes up for losing the least interesting 8 bits of your IPv4 address.
This feature isn't designed to provide actual anonymity. The documentation even suggests the feature was designed to minimally satisfy certain legal or contractual obligations:
>> This feature is designed to help site owners comply with their own privacy policies or, in some countries, recommendations from local data protection authorities, which may prevent the storage of full IP address information.
Notice that this mentions pre-GDPR "recommendations" and that compliance is the goal, not user anonymity.
(side note: that documentation doesn't even acknowledge IPv6. Does the aip=1 feature even exist for IPv6?)