In the specific case of core-js I think the author was in dire need for financial support after an accident - github.com/zloirock/core-js/issues...
Again, it seems more like a cry for help instead of a grab for money. Situations may arise when reaching out is the only option - the alternative would be to abandon maintaining the project completely. And very few developers are also marketing-savvy enough to find alternate sources of income that will allow them to continue their work.
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