It seems as though you want things both ways.
The notion that an experienced person should automatically consider mentorship to be part of their job is not backed by any code or contract that I'm aware of. It's just increasingly been pushed on people who haven't pushed back enough to keep a new generation from feeling as though they are entitled to it.
You can sample as large a pool of senior devs as you want; I suspect that if you can find 1-in-10 who have specific wording and structure in their employment contracts about the percentage of their time that they should expect to allocate to mentorship, you'll be beating my own estimations. No, what you're doing is attempting to codify a very polite form of exploitation. Yes, those people are usually among the most well-compensated, but it doesn't change the fact that they have their own job to do and a finite time in which to do it.
Please note that I am not suggesting anyone on a team could somehow work in a vacuum. Also, even the most experienced person frequently needs to ask for help, guidance and clarification. Being experienced is pretty much the opposite of knowing everything and is much closer to a measure of how much you have forgotten.
Nor am I implying that senior people have any business being rude or dismissive towards team members who legitimately have good reasons to interrupt someone's flow state. Timely clarification is important to a project regardless of who is doing what.
What I am saying is that if you run a company you might legitimately be horrified to learn that your most critical team members are spending 70% of their productive time mentoring, especially if that remaining 30% of their time is so fragmented that it never resembles a true flow state. You need those people to Build The Thing.
If you don't get that, then you're not living in the same reality I am.