It’s a mailshot and not everyone has had it so if you chat with people who have had it, you might be able to figure out the mailshot criteria
The relevant poeple to receive it in my view would be:
anyone who was on the old CMU - they will have missed a year of cotisations because CMU ceased to exist when PUMa came in. They won’t pay the same cotisations as before and maybe even none at all because under PUMa, pension income isn’t taken into account, only capital income (capital gains and rental income).
anyone who has earned income or business income below the lower threshold of around 4000 euros but also has capital income above the higher threshold of 9654 euros per annum and nobody else in the houshold pays cotisations based on income higher than that threshold will now get a supplementary cotisations bill.
This only affects people ‘a la charge de France’ for healthcare - so people who are in the system with an S1 from the UK won’t be liable for these cotisations, just as they weren’t liable for CMU cotisations.
I’d have thought they’d only mailshot the above people and not include people on S1s so if anyone with an S1 who isn’t working (which they shouldn’t be, as if you’re working you shouldn’t still have the S1) has received it, it’s obviously an error. I’d have guessed that the error might be that they may have mailshotted anyone with capital income above the relevant amount, using info from our tax returns - except if that was the case I’d have had the letter myself and I haven’t!