Right now as far as I know, SE stores the IP addresses used to access any SE site forever. Every moderator or employee with moderator powers can see a list of every IP address a user ever accessed the site from.
IP addresses are considered personal data under the GDPR, and I think it is worth discussing how long they should be stored. They are often considered not particularly sensitive information, and in many cases they aren't. An IP address from a mobile phone provider won't tell much about you, and unless you can access or subpoena the provider, knowing the IP address doesn't really reveal anything. But there are also many different cases, and in some cases IP addresses can reveal a lot. If you access SE from a company computer, the IP address could identify which company you work for. They also can give a rough location of where you live and where you travel.
There is a legitimate need for SE to store IP addresses and to make them available to moderators. But keeping all the information indefinitely is a bit harder to justify. I could also imagine treating "well-behaved" users differently here than users suspended for rule violations that involved sock puppets. The curious thing here is that the users that get the most privacy are actually spammers, as their accounts are destroyed entirely.
Is it really necessary to store IP addresses forever, or would a shorter duration be enough? Alternatively I could imagine simply restricting the visibility of historical IP addresses, as there isn't really a need to look them up on their own.