* Germany has an Obamacare-like system: Insurance is private, but mandatory.

* You can choose and change your insurance provider freely, and they cannot reject you

So if that is true, health insurance companies have the risk of older residents joining, whether or not the previously resided in Germany, and it's not like they benefited from their contributions before.

So how exactly would this exclusion work? Is insurance not mandatory for immigrants after a certain age? Do insurance companies have a right to reject them?

I was not able to find a direct discussion of the question, but this page discussing the health problems of older immigrants (including "access to the health care system") does not mention the possibility that they would be excluded from the health care system altogether: https://heimatkunde.boell.de/de/2013/11/18/ältere-migrantinn...