Steps to reproduce:
This won't affect most users, because most users won't want things in that list. But making the setting 'sticky' will help wikignomes a lot.
We intentionally made this not sticky to ensure that the editor is always in the same state when you open it, to avoid people getting confused. This is in line with how Word etc. do it, though I admit we don't have bullet-proof evidence it's absolutely required. However, I'd need to have some evidence to reverse this decision. (If you have some, please re-open.)
I've had second thoughts about whether this will affect most users. Earlier, I was thinking mostly of people typing in English from an English keyboard.
Now I'm thinking that we either need to change this behavior, or we need to move the Special Characters inserter into the upper part of the menu. With the current placement with the current behavior, and when working in a language like Breton or Welsh, most users will be required to click on 'More' for nearly all content contributions.
This doesn't seem OOUI related to me, it's either
I think this is the "a per-user saved setting for menu preferences". The Special Characters item has already been moved out of the menu (for mostly unrelated reasons).