I suppose it's possible the poster's IP is stored in the post metadata itself, along with the date, username, and so forth. Don't know if that's how Reddit works but in that case it'll be retained forever.
As for the reason for 7 years worth of data, I think that's because the firm wants to go back and search for infringing acitivity on all those IP addresses due to dynamic IP updates and so forth. If the user is still active on Reddit the more recent IP addresses probably are enough to identify him if a warrant is served to the ISP. The ISPs I know only retain a link from a dynamic IP to a subscriber for one to two years max.
Yeah that's why it's good to start a new reddit account every year and delete the old one. As well as VPN, regularly changing mac address on your router etc.
a lot of cable companies will have you keep ip addresses for a very long time. The only way I've found to get a new one quickly (without taking my cable modem offline for many hours) is to swap the mac of the router in ddwrt (router firmware). That works "nearly instantly" as long as you reset your router and cable modem.
Or if you're like me, Reddit will helpfully shadowban everything you ever wrote on your decade-plus account, while simultaneously claiming you can't use the appeals page because the account is still in good standing.
As a bonus, anything anybody ever wrote in response to me is gone too.
As for the reason for 7 years worth of data, I think that's because the firm wants to go back and search for infringing acitivity on all those IP addresses due to dynamic IP updates and so forth. If the user is still active on Reddit the more recent IP addresses probably are enough to identify him if a warrant is served to the ISP. The ISPs I know only retain a link from a dynamic IP to a subscriber for one to two years max.