I think this is only the case with the free version. Source: I worked with some big German companies which wanted to have an autotranslate function for their software. DeepL was the top choice, because the quality is very good and the data protection agreements for the paid service left no questions.

EULA they collect all what you type there and use it for training their algorithm

This isn’t the case for some specific corporate contract for the pro version. But, it’s not publicly available.

Interesting thought about the visibility