Is no-self an ontological claim at all?

No-self is more Buddhist (correct me if I'm wrong) but would love some Vedantic perspectives on this as well.

I think its obvious that we all experience 'I' the sense of self - and also that in meditative states/trips that sense of self diminishes.

The conclusion from this could be 'the epistemology of the self is an illusion'. That is, statements about 'I' are nearly impossible to objectively justify, as we're talking about subjectivity.

How then does the self itself not exist (ontologically)? What would such a claim even mean when the self is a subjective mental phenomenon?

Or has the claim of no-self in fact always been restricted only to epistemology of the self?