...well crap. Time to update my side projects to handle this new flow. I wasn't aware this was even coming. Looks like there are a lot of exemptions (my side project costs customers less than 30 euros), but it's ultimately up to the customer's bank.
I guess I'll move to Stripe checkout instead of my custom form.
I built Quaderno to deal with the VATMOSS nightmare on Stripe. More information at https://quaderno.io/integrations/stripe/. Let me know if you have any questions. Glad to help.
I've tried Quaderno in the past for a simple SaaS I built but found it didn't really work, it was a while back so I may be misremembering but I believe you couldn't use Stripe Billing's built-in free trial system with it, because the Quaderno checkout would create a new customer.
I guess I'll move to Stripe checkout instead of my custom form.