I agree with the OP that board owner should be able to anonymize IP's, There is very seldom a need to store the posters IP for the whole lifetime of a post and it's against the "need to know"-principle.
With standard phpBB, it's quite easy to find out the IP the post was made from, the ISP where that IP belongs to (via whois service) and maybe (if it is a static IP) eg. the company network. It's just in the post information area, giving "IP", "posts made from this IP", "Other IP used by the user", all IPs with a link to the whois service.
Apart from that, there are some legal calls for "telecommunications data retention
", which often affects ISPs, but normal board owners should very seldom have legal needs to store communications data eternally.
For a limited period it is useful to check session keys, or to identify multiple accounts from one user (there may be other usecases as well). But storing the data forever is not very useful and maybe against the law in some country or against board owners principles