So it looks like at some point I might need to cull registrations which never log in. Have to wait and see. That's another question for sometime. Do I cull registrations where no posts have been made, and no-one has logged in beyond a certain time frame? Not essential to answer now, but will become a question since I cant tell a spam registration immediately now and so are not deleting them.
There are 1736 members currently. Of those, about 950 have never posted a message.
Some may be lurkers, but lurkers tend to log in, especially if they want to read content from areas that require login or membership.
Checking some of those 950 at random show that some of them have not logged in again since they registered. I'd say if someone registered in 2018 or 2019 and hasn't logged in since registering, they are probably not coming back. Maybe they are reading without logging in but it seems doubtful.
I know some forums purge after twelve months of inactivity; that is, no posts or logins. I had that happen twice on another writers' forum. They purged without notification either. So it was like you get busy, go back and, poof!, you have no account and have to start over. That forum is long gone now.
So, probably the best thing would be to set an inactivity period. A year is okay. After six months of inactivity, send an eMail--this can probably be automated--saying we miss you and want you back and inviting them to come back. After a year, send an eMail warning them that due to inactivity their account is subject to deletion, but all they have to do to keep their account is login or send you an eMail.
If there is no response, or the eMail bounces, you can probably purge them.
Of course, before implementing any of that, change the terms of service to warn people that their accounts are subject to deletion after a year of inactivity.