Good day! For example, I have some plugins, can I add a text and link “Powered by ZS1994” visible for visitors of my buyers web-sites? Thanks
If buyers have the ability to remove it, yes. If they don’t, definitely not, if you add it post-purchase by some server script to existing buyers, you’re 100% going to get in serious trouble.
You can add “Powered / Made by” but insert a text in the docs on how to remove it, since this will be a pretty frequent question you’ll be getting! 
Cheers mate!
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thank you so much for your detailded answer! 
good luck to you and have a nice day!