Fifty's opt-out page speaks only to Californians, from its heading to its hotline. People elsewhere may still have rights that apply to Fifty.
In the UK, ask for erasure under the UK GDPR. Fifty Technology Limited is a British company registered in London with an appointed data protection officer. If Fifty doesn't resolve your complaint, contact the Information Commissioner's Office.
In the EU, ask for erasure under the EU GDPR. If Fifty doesn't resolve your complaint, contact the data protection authority in your country. National authorities handle complaints about GDPR violations.
Elsewhere in the US, the opt-out page offers you nothing, but the privacy policy does. Section 11 names Colorado, Virginia, Connecticut, Utah, Florida, Nevada, Oregon, Texas, Washington and Montana as states whose residents may have additional rights under their own privacy legislation, exercised through the same email and hotline a Californian would use. Cite your state's law by name when you write, and send the same email regardless. A clear written request costs you ten minutes.