Can confirm. As a dev of an extension with 10k users I get 3-4 emails a month in my spam which ask me to monetize my extension by secretly changing its users' search engines. My extension is open-source and quite small, but if the change was sneaked in I think most of the users would not notice. I stick to using userscripts for the most part since you can easily check their downloaded source and disable updates.
Example:
Beth Anderson <beth@monetize-extensions.com> Mon 10:58 AM To: Mostly Spam <dev@x-ing.space>
Hello
I am Beth and I am offering monetization for browser extensions, with everything that is going on our team was extremely focused and productive in creating a way to earn revenue on extensions.
We offer to change default search to Bing or Yahoo on your extension which can earn up to $800 a month per 5000 users. This is a premium product by invitation only and can easily be added to your chrome extensions.
You are might curious to know if it is allowed? And I must say that this is completely allowed! Please reply to this email to discuss this further!
Looking forward hearing from you!
Beth Anderson
Business Development Manager