This question united kingdom - UK EU settlment scheme, effects when acquiring British citizenship was migrated from law to expatriates. (The original title was: UK EU settlment scheme, which is a bit misleading).
The main portion of the question was asking about the effects to their German and Polish citizenships after a possible acquirement of British citizenship.
Such a question can only be properly answered based on nationality laws and in this case also a special Brexit transition law.
It would be better if such questions remained in law and not be migrated.