Find columns which have all constant values

Generate test data:

clear;

n = 10;
c1 = repmat(3,[n,1]); % n times of value 3
c2 = rand(n,1); % n values
c3 = repmat(NaN, [n,1]);
c4 = c3;
c4(4) = 2;
c5 = repmat(missing,[n,1]);
c6 = cellstr(repmat('test',n,1));
c7 = c6;
c7{7} = 'hello';
c8 = repmat(false,[n,1]);
c9 = c8;
c9(9) = true;

T = table(c1,c2,c3,c4,c5,c6,c7,c8,c9);

cellData = table2cell(T);
constantColumnIndex = constantColumnExtraction(cellData);
constantColumnNames = T.Properties.VariableNames(constantColumnIndex);