I don't know why the devs never thought of this... Remember when they deleted profiles that were 90 days old and older? Why don't they do the same with clans? It'd be dumb not to. So why not delete clans that are 90 days and older. That's all I have to say. Doesn't need much more effort.
You're saying 90 days and older. Do you mean 90 days inactive and over? It's 6 months for profiles anyway. No support. How would this be measured? A system that reads the activity of players in the clan would need to be created. It isn't needed.
I think aftera certain period of clan inactivity, you are able to take a clans name by emailing support or devs will delete the clan or something. Don't hold me on this though, it is just something I heard through the grapevine.