@kzimmermann Mastodon, by default, keeps IP addresses for an entire year, which basically tracks each user's IPs, as long as they've been used within 12 months.
This was made configurable by me recently, but there hasn't been a new release including that change.
@kzimmermann (of course, admins can choose not to record IP addresses by not passing over the real IP from their frontend proxy in a header at all. still.)