I'm not sure why they're sending you a warning, but the type of subscription matters. If you're referring to the legacy subscription that I set up originally, you should have canceled that ages ago anyway. I fixed the code so that it recognizes the old V1 license key purchased from Google Play to Grant you lifetime access to the app without having to maintain a subscription.
If you are talking about a regular monthly subscription, then it's telling you exactly what is the case. So long as you have the monthly subscription running then you get full access to the app and all its features. However, if you do not renew the subscription and allow it to lapse then you will only be allowed to use the free features of the app.
If you don't want to keep paying a monthly subscription there are other options, as you could get a yearly subscription or a lifetime subscription where you pay once and never worry about it again.
Perhaps I haven't fully understood what's going on here, so if it seems I haven't, please explain in greater detail what you meant by this warning and also tell me which subscription you originally chose.