Back in the day I was able to get my twitter member number. It may not have been displayed, but I think I found it in their api. Fairly certain is was just basically a counter for when you signed up.
Something like that is more than good enough for me. But you may want to promote sustained membership or something like that. So I think that filtering out those who, for whatever reason, stopped subscribing would be ok. To me, paid membership or not doesn't really matter. I mean, I didn't pay for the first period anyways, but I kept using kagi, cheering you guys on. And I happily started paying once that was possible.
I am just curious how early on I was. And I think for us, early adopters, that would be nice to know.