Some larger companies do this in disaster drills or other emergency preparations as well. You give the team the task to stand up a database cluster from scratch and pull a dataset into it - and none of the seniors may be contacted. This is pretty effective at rooting out such knowledge silos. And it can build confidence in the other team members once they manage to do it. The main drawback is that it's just non-revenue generating preventative work.

As an IT/Admin/SRE team, we've also found that training isn't just good for the industry, but it also helps to keep such knowledge islands in check. I currently need to make sure that my junior is able to take over most crucial daily tasks, and in the future, all of my important technical tasks - and he's doing wonderful. Though that's now a staffing thing, which costs money.

In the end, it's a question how sustainable a company wants to be long-term I think.