This frequency is protected because there is a gap in the absorption spectrum of our atmosphere. This is not the same for every atmosphere, so other planets will have different gaps (unless the composition of gases is similar enough), and thus it won't make too much sense right now to single out this frequency for other civilisations. AFAIK we don't broadcast, because it would interfere with astronomic measurements.
Not true. While some of the radio spectrum has been reserved specifically because of the lack of absorption in the atmosphere, the 1420MHz line is the famous 21cm line frequenctly measured by cosmologists to look at the epoch of reionization. It is due to the hyperfine structure of hydrogen, and would not be of unique importance to our civilization.
However, we don't broadcast anything. Any broadcasts from earth would hugely overshadow any small signal we're trying to read.
