That they only registered a few votes suggests the following causes. Canvas users, for whatever reason, are not too committed above and beyond the cause, to investing their time and efforts into searching for improvements; that Outcomes are not popular, possibly because the concept is at best, noble in principle, but something that could have been thought through better. Since the community clearly gets behind ideas they believe in, my guess is the lack of votes is not apathy but the latter.
The current status is that Canvas doesn't permit the deletion of Outcomes that have assignments already submitted and neither can they be edited. I get the rationale, since no-one would like to create an Outcome, only to find that a colleague has deleted the same or edited out one's hard work. But why can't Outcomes be unpublished or hidden as a smart view (see this idea https://community.canvaslms.com/ideas/8973-gradebook-and-learning-mastery-bespoke-smart-views ) or edited as a new instance of the origional? Permit me to explain this latter idea further.
Other software platforms use this concept. For example, Planet eStream permits only admin the option to edit or delete original videos that have been uploaded for embedding into a VLE such as Canvas. Teaching staff have a lower level of permission that permits them to create as a "new instance" of the origional. The origional video remains in tact, but teachers can redeploy the same, edit out unwanted material and add interactive quizzes to the timeline. Even if all this was achieved, I'm still not convinced this would be the definitive solution to get buy-in from all users. May I suggest another idea related to this:
Perhaps the mastery concept with it's 4 calcuation methods is something peculiar to the US education market? The idea is certainly a little strange to the UK FE sector and our teachers are still getting their heads around it. Why is this? Well for starters, why would average (mean) not be a consideration as one of the Outcome calculation points? Secondly, Mastery appears to be an uncessary add-on to Grades.
Why could Grades not have a colour coding (layered under or over the scores or grades; emphasis added) instead of in Learning Mastery ? A colour schema could be deployed in the same way Microsoft Excel offers the application of colour scales or colouring cells. That would render Learning Mastery obsolete and it would reduce the amount of work needed. The current setup dictates that if one requires to monitor Outcomes for the exact same criteria set out in the Rubric, an Outcome has to be deployed each time. This is surely duplication? Mastery (or moreover the colour schema applied to the grades) should be aligned to the Rubric such that when the Rubric selection is made, the score reports to the Grades and a colour is overlaid according to the schema implemented.