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Since you are actually representing the same information with half of the data this is in fact a compression. It is called "point compression" in the SEC1 standard. Please refer to paragraph 2.3.3 of secg.org/sec1-v2.pdf
A public key is a point in a curve. In order to recover the point is sufficient to have only one between x and y (typically x is chosen) and the sign of the other.
Epoch-based consensus algorithms such as babe and aura require to periodically produce blocks in order to allow to trigger some events at the end of each epoch (e.g. for babe => authorities set and rand seed change). This information is embedded within the block using the so called Digest entry.
