What is the general goal of the feature?
Allow the user to find select multiple options in a large list easily.
What can you contribute to making this feature a reality?
I'm not sure why this was not implemented at the same time as search in select one. One issue compared to select1 would be that if you are filtering the choices then the shown list may not include all the selections you have made. This would not be desirable. One solution to this could be to show 2 lists on the page.
List 1. All choices filtered by the search criteria with both checked and unchecked choices shown. The user can check and uncheck choices in this list.
List 2. Only the selected choices. Users can un check from this list after which this choice will be removed.
Possibly the above setup would also be a good UX for select multiples that don't have a search filter. Ie if you have 30 choices and you have selected 3 it might be nice to see those 3 selected choices in one place without having to scroll up and down and try to get your eye to follow across from the checkbox to the text to verify that you have selected what you want.
I just tried the select_one with search appearance and it filters regardless of the selection. I think it's a bug that this isn't implemented for select_multiple and I'd be OK having the same behavior.
I thought/hoped that this would be a really simple thing to add but unfortunately there are some strange things happening around select appearances that I'm still trying to sort out. Specifically, at some point a search() function was introduced as an appearance to filter CSVs. Later, a search appearance that lets users view a subset of select one choices (what you're describing) was added, blocking the search() function. That search() function is still accessible for select multiples so I think we should try to go through some kind of formal deprecation process for it (announce that it's going away a couple of revisions before taking it out).
Additionally, there was some confusion about the name of the appearance that lets the user do the filtering (see discussion here) so now both search and autocomplete are accepted.
I think it would be good to build the multi select autocomplete widget as @Neil_Penman describes. For now it should only respond to autocomplete as we figure out what to do with search. Issue filed at https://github.com/opendatakit/collect/issues/1373