It is free to run your own apps on your own device and has been for many years now.
There are limitations. The signature on the binary is only valid for 7 days I believe, so naively you'd have to rebuild/reload every week, however there are certainly automations that do this for third party apps with a little daemon running on your Mac in the background, and I suspect there are equivalent automations for your own apps/open source codebases to reduce the hassle.