Yes, there are a couple of reasons; 1. Like you mention, if you don't receive a subscription in the last 6 months, it's probably not something you're bothered

Yes, there are a couple of reasons; 1. Like you mention, if you don't receive a subscription in the last 6 months, it's probably not something you're bothered about. 2. Scanning some users inboxes is very bandwidth intensive, and time consuming (some people never delete an email). We experimented with 1 year scans and some users just give up waiting. We could probably improve the process, however due to point 1, we think 6 months is probably adequate.