Minecraft wiki entry for Development_versions, associated text: Far more blocks can now be placed on the tops and sides of transparent blocks including glass, all forms of ice, glowstone, active redstone lamps, all forms of stained glass, sea lanterns, beacons, jack o'lanterns and leaves on fancy graphics.
A list of all such changes across 1.14's snapshots can be seen at Java Edition 1.14/Transparent block changes.