I am not saying this is a solution, but the reason you see this is because a number of companies, such as TikTok and LinkedIn, have been caught scraping the clipboard.

https://www.forbes.com/sites/daveywinder/2020/07/11/iphone-u...

https://www.forbes.com/sites/zakdoffman/2020/06/26/warning-a...

That said, for web applications, you are required to do things like use HTTPS for the modern clipboard API, which provides the user some guarantees. Not much works without HTTPS anymore though.