It seems like it's a subscription from what I can tell. I see some people buying 'lifetime' subscriptions in the play store reviews, so I'm guessing that's the case. Now, it also looks like it's from one lone developer with 27 contributors in GitHub. The app can be downloaded from GitHub (I would use obtainium for that), so that's a good thing. I think supporting developments like this via a subscription (assuming it's not crazy expensive) should be normalized if the product is good enough. On that factor, I can't really comment since I just KeePass the hell out of my credentials over Syncthing, but I do believe there's value in supporting developers like this guy.

https://github.com/AChep/keyguard-app