Percentage of missing values so that we can't perform imputation
In my opinion that depends on the context, like the question you want to answer, and also on the domain. 

If the remaining absolute data set is still big and diverse enough to allow conclusions about that feature, the percentange of records that can not add to that imputation is not a problem, in my understanding.

It might also be that features sometimes are not just missing, but  do not apply to the particular kind of record.