I decreased the priority because it turned out it can be set as a server startup option (it was not obvious right away due to MDEV-8220).
I still think it makes sense to allow it to be set globally, so I will keep it open; if there were really good reasons to prohibit it, please feel free to close as 'Won't fix'.

Please also document it as a server option, currently it's not there: https://mariadb.com/kb/en/mariadb/full-list-of-mariadb-options-system-and-status-variables/

This was intentional and now you can also use server option. What would be reasoning to be able to set it dynamically as server wide setting ?

> What would be reasoning to be able to set it dynamically as server wide setting?

Same as usual: changing (fixing) configuration without having to restart the server, it's important for many production setups, people tend to complain about not being able to change something at runtime. Generally, I expect a variable to be static only if making it dynamic causes potential troubles or makes the logic more complicated. If that's the case here, please by all means close it as "won't fix", otherwise maybe you'll re-consider.

http://lists.askmonty.org/pipermail/commits/2015-June/008080.html

commit d535728165acb2eb55140bb70fa44c458d1ccc06
Author: Jan Lindström <jan.lindstrom@mariadb.com>
Date: Fri Jun 26 11:42:09 2015 +0300

MDEV-8219: enforce_storage_engine cannot be set globally

Change session only variable enforce_storage_engine to be
session variable and make sure that also global value
is used if session variable is not set.