Is AirBuddy 2 a subscription?

No.

AirBuddy 2 is a one-time purchase that gives you access to all of the app's features and at least a year of free updates with new features and bug fixes.

Will it ever be a subscription?

I can't promise anything with regards to future plans. What I can say is that I'm not a fan of the subscription business model and I have never considered (and probably will never consider) making AirBuddy a subscription.