However, such a thing could be relatively easily added to X11 without changing the X protocol, so this does not appear as a sufficient motivation for the existence of Wayland.

I have not tried Wayland yet, because I have never heard anyone describing an important enough advantage of Wayland, while it definitely has disadvantages, like not being network transparent, which is an X11 feature that I use.

Therefore, I do not know which is the truth, but from the complaints that I have heard the problem seems to be that in Wayland it is not simple to control the access rights to windows and clipboards.

Yes, access to those must be restricted, but it must be very easy for users to specify when to share windows with someone else or between their own applications. The complaints about Wayland indicate that this mechanism of how to allow sharing has not been thought well. It should have been something as easy as clicking a set of windows to specify something like the first being allowed to access the others, or like each of them being able to access all the others.

This should have been a major consideration when designing access control and it appears that a lot of such essential requirements have been overlooked when Wayland was designed and they had to be patched somehow later, which does not inspire confidence in the quality of the design.

My question was "How often has that been a problem. Is it a vulnerability that has been, or or likely to be, exploited in practice?

I would have thought that filesystem access is the biggest issue, followed by network access. There are solutions for these, of course, but in most cases the default is either unrestricted or what the app asked for.