No, a wither being inside of unbreakable blocks and dying via suffocation isnt a glitch

Bruh don't be sarcastic he's asking a legitimate question and was hoping for a legitimate answer. He's probably a new player looking to set a new record or at least try. Imagine he's socially anxious and had to pull his guts together just to ask this, and you give him this. Then you wonder why people submit runs with mistakes that go against the rules.

yes it is consider a glitch @Tommaconda