The autoAssigned and weight attributes mentors can have both share a similar purpose, as both of them affect how many mentees a mentor has. The difference is that weight (T287917) affects how many mentees the mentor gets (but they still receive some mentees), but autoAssigned is boolean: either you are assigned to any mentees, or you are not.
Both flags are even presented to the user inside a single weight dropdown:
In other words, autoAssigned: false is equivalent to a weight of zero. Since there is no technical reason for storing those two values separately, let's migrate them into one.