Deleting columns containing more than a particular number of missing values is one of the techniques of dimensionality r

[answerhappygod]

Deleting columns containing more than a particular number ofmissing values is one of the techniques of dimensionalityreduction.What percent of values should be missing in the column to drop itcompletely??
I know sometimes whether a column can be dropped will depend onwhether this dependent variable is important to the whole analysis.What test should we use to determine whether the dependent variableis important or not?
Thank you!