Well I think all physical observations of the universe indicate that it proceeds deterministically. So, unless there are deeper non-deterministic processes, I think that means free will is impossible. If free will is impossible then I think it might be fair to say one 'experiencing' free will, whatever that means, is experiencing an illusion.
That said, just because something is an illusion does not mean that it does not 'exist' in a human sense. Consider sight, for example. The information gathered by your eyes, just a slice of the electromagnetic spectrum, is constructed by the brain into the experience of vision. This experience is nothing more than a hallucination really. The human eye does not capture complete information and on top of that the brain imperfectly generates vision from that incomplete information. Nothing exists as you see it. Your vision just corresponds to things that exist in a very limited way. After evolving for millions of years, the illusion has gotten pretty nice by now.
Here maybe we have a bit of a paradox. Vision is a hallucination that nonetheless helps one to make sense of the world. Sometimes I think free will is something like sight. One can intellectually understand that it is an illusion at the same time that denying it would be something like living your life with a blindfold on.
Idk. Prolly just a cope. :/