i am also flying to pmi today and was asked to upload docs. You don’t need to fill in the Spanish locator form anymore unless unvaccinated. I do this route monthly and always upload my vax certificate as BA may not let me check in online ( these days no luggage for obvious reasons so want to ensure I can go straight through to the lounge). It only takes minutes for BA to verify docs. You would not be denied boarding in the event you didn’t do it though. Bear in mind mask wearing in taxis or public transport is still mandatory in Spain . Spain are still clinging onto the covid rules.
Is ba.com just out of date or is this genuinely compulsory?