This is the one question we get asked more than any other.
Is tyre stretching legal?
The short answer is, there is no real answer. Different countries around the world have different laws, and those laws are open to interpretation in different ways and may or may not have been clarified by a court ruling.
Most countries will state something along the lines of 'tyres should be of the appropriate size and within manufacurers limits for the rim its mounted to'.
The safest thing to do is to seek advice in your country from a qualified tester or member of local law enforcement who knows the laws near you.
You can read more here https://www.torquecars.com/styling/stretched-tyres.php but as always, this site is designed to showcase different sizes. Many of the setups you see are for show only so please seek advice first about the laws in your country.