Mutils often add new rules during league evolution (cf CSB).
When trying to help new user with a particular problem, I sometime miss the possibility to regress to previous league to be able to see what he/she see and be sure of the exact statement for league.
I may be useful to allow one to temporally regress from high league to lower ones (without possibilty to submit of course) in order to allow us to help others more efficiently. We can already fight against them but this may not be enough.
I agree. If we regress though, we should be put under the same rules and restrictions that the league provides (CSB inputs revert to the “simple” version when regressing to silver, CoIF doesn’t have towers in wood leagues, etc). This makes sure the opponents don’t break from some new type of input, allowing us to make a (mostly) fair evaluation of their performance.
I see league demotion more as a tool to start from scratch again rather than a tool to check out rules from lower leagues. But, yes, that would be useful.
However, I’m afraid the priority of such a feature is very low, because of its technical cost and low impact for users. So I don’t see it being planned any time soon.
If your goal is to play with lower leagues rules, you just need to select a lower-league opponent in the IDE. When two players from two different leagues face each other, the referee automatically uses the lowest league rules. (EDIT : that’s wrong, see other post below)
The main problem is that there is no way to pick someone out of top 1000 on any multi, due to API restrictions 
So in CSB you cannot pick anyone below gold league.
I’d personally use it when helping out someone in a lower league than I am. Currently, I have to make an alt and hope I don’t get promoted while helping.
It seems that I misunderstood how the game engine actually works. The engine picks the league of the player who PLAY the game (If I’m bronze and play against a wood player, bronze rules will apply). My bad … TIL
The goal is not to check rules from others league but to be able to help noobs ! I do not propose here to allow REAL regression (with a submit) but only to be able to allow regression in IDE. From this point of view all you need to do is
1- just forbid submit by deactivating submit button in order to simplify things)
2- Add a “regression button” (or use the “League Level” menu)
3- Inform the IDE of the league used (probably already done for technical reason).
I do not see any massive work here.
Btw, if CG do not have time to do such modification may I remind you that there is more or less 500 000 users in the community and that, probably, allow them the pull-request feature of github may be a good idea.