*Creating* plausible issues is hard work, I assure you as a university
professor.  Coming up with "exercises" that are not makework requires
expertise in both the domain and in educational psychology.  (Some
people are "just good at it", of course, but it's quite clear from
popular textbooks that most are not.)  I think that would be a very
unproductive use of developer time, especially since "git clone; git
checkout some-tag-in-2017" is pretty much what you're asking for
otherwise.

The problem is not a lack of issues to practice on.  It's that (1) the
PR process itself is a barrier, or at least an annoyance, and (2) many
new contributors need mentoring.  (Or think they do.  Some just need
encouragment, others need help on technique, but both groups are more
or less blocked without the mentoring.)

And, of course, real contribution involves a lot of unfun work.
Writing tests, writing documentation, explaining to other developers
who start out -1 because they don't get it, overcoming your own mental
blocks to changing your submission because *you* don't get it, and on
and on.  A lot of newcomers think "I'm not good at that, if I have to
do it I can't contribute" (and a few selfishly think they can just do
the fun parts and achieve fame and fortune), but you know, "if not
you, then who?  If you don't do it for Python, where are you going to
be able to contribute?"

To be honest, although I'm not a specialist in organizational behavior
and am operating with a small sample, I can say that from the point of
view of identifying tasks, finding solutions, and implementing them,
Python is the most effective non-hierarchical organization I've ever
seen.  I can't say I've seen more than one or two hierarchical
organizations that are significantly better at implementing solutions
and don't burn up their workers in the process -- and the ones I'm
thinking of are way smaller than Python.  (Yes, I know that there are
people who have gotten burned up in Python, too.  We can do better on
that, but Python does not deliberately sacrifice people to the
organization.)

I have to point out that there's a whole crew over on corementorship
doing this work, and at least one Very Senior Developer with their own
private mentoring program.[1]  IMO, that is a big part of why Python
is as successful as it is.  If more senior developers would take on
these tasks it would have a big effect downstream.  But emotional work
is hard, and it comes in big chunks.  In many situations you have to
follow through, on the mentee's schedule, or the mentee will "slip the
hook and swim away."  So it's a big ask.  I'm willing to make that ask
in the abstract, but there's not even one senior developer I'm able to
point to and say "definitely that person would do more for Python by
mentoring than by hacking".  It's a very hard problem.