While you can get a chip-enabled card reader to accept a magstripe-only transaction by altering the magstripe, the final authorisation decision is with the card issuer.

And the card schemes are mandating the issuers to keep track whether a card was issued with a chip and then reject authorisation attempts with a magstripe that claims "no chip" available.

That being said, in reality there are again and again still issuers who did not implement this and similar security measures [1].

[1] https://krebsonsecurity.com/2020/07/is-your-chip-card-secure...