Three points:
It’s a tax and it looks to be computed as a percentage surcharge of income tax. I am not sure about Germany in particular, but ordinarily people who are poor pay less tax or no tax in general.
The interdict will be based on a formal and written renunciation of Christian faith. People who reject Christ should of course be denied the sacraments of Christ.
The leaked document either does not speak to the cases when tax is not paid and the apostasy is not forthcoming, or that part was not printed by Die Welt. I find it quite plausible that inability to pay will be treated in some charitable ways. The problem peculiar to German Catholicism is not cases of poverty but people who are Christmas and Easter Catholics and can pay, they just don’t want to. I think it is reasonable to ask them to clarify if they actually have any Catholic faith left.
Thanks, those are good points.
I'm sure we will see more details in coming days