Um, you just create a cert in XCode, pair your device to it, load the certs onto the device and run the app on it.
You're not going to do it on a Linux box for sure as the tools aren't there but you certainly can run your own stuff on iOS. You don't have to sign up for anything either - just install XCode from the app store and build stuff.
Random anecdote: if you don't pay 99€ yearly for your developer subscription Apple revokes your certificates and disables your apps on the Apple app store. Your problem indeed.
You don't have to pay Apple to deploy to your own device. I don't have a developer subscription. You only have to pay the 99€ to access the store and provisioning.
You're not going to do it on a Linux box for sure as the tools aren't there but you certainly can run your own stuff on iOS. You don't have to sign up for anything either - just install XCode from the app store and build stuff.