I'm new to X11 and want to understand if it is really as dangerous as they say on the Internet.
I will explain how I understand this. Any application launched from under the current user has access to the keyboard, mouse, display (e.g. taking a screenshot), and this is not good. But, if we install programs from the official repository (for example, for Debian), which are unlikely to contain keyloggers, etc., then the danger seems exaggerated. Am I wrong?
Yes, you can open applications on separate servers (for example, Xephyr), but this is inconvenient, since there is no shared clipboard. Creating a clipboard based on tmp files is also inconvenient.