Is there any historical evidence that sysops being
able to edit JS / CSS caused some serious issues?
Oh yes, this happens more often than I feel it needs to. I remember a
situation when I posted a fix for a script in the MediaWiki:…
namespace as an {{edit request}}, and a well-meaning administrator
tried to "improve" my line of code and forgot a comma, breaking all
JavaScript for all logged-in as well as not logged-in Wikipedia
editors and readers for some painful minutes.
I believe such can be avoided with more clear roles that are visible
for everybody. A separate "tech admin" role would also allow
volunteers to apply for exactly that, and not be asked why they don't
do enough "administrator actions" with their privileges.
Sure, this is anecdotal evidence. Please forgive me, but I currently
don't have the time to find the pages documenting these situation.
Best
Thiemo