In QGIS 3.6.2 (on Windows 10), one can open the properties of a layer, click on Metadata, and enter information about the layer. At the bottom of the properties window is the option to save metadata as a file (a QMD file) or "Save as Default".
Where are default metadata stored for PostGIS layers in @qgis 3.x? It's not in the project file because I didn't save a project file but when reloading layers from PostGIS, the metadata changes are preserved.