These combinations of blocks can no longer be made in current versions.
Stationary water[]
Prior to 15w51a, melting ice blocks would not cause the water to start flowing until it received a block update.
Water in the Nether[]
Ice blocks can be collected with Silk Touch in Beta 1.9 Prerelease 4 and 5. If placed in the nether and melted in a version prior to 12w17a, they would become water, resulting in a legitimate way to obtain and manipulate water in the dimension.
Wet sponges in the Nether[]
Prior to 19w36a, wet sponges can exist and be placed as normal in the Nether without drying out immediately.
Snowy blocks without snow[]
Prior to 1.16.2, endermen could pick up snowy grass, mycelium and podzol blocks. Upon placing these down, they would still have the snowy block state set to true, but there would be no snow on top. Indeed, endermen which are holding these blocks in the first place (alongside other things, such as netherrack, or indeed any block from around their introduction) are also a discontinued feature.
Old wall configurations[]
Before walls were updated to be cleaner in 1.16, placing them in a vertical sheet would cause many gaps, and there would also be gaps with full solid blocks placed above them. These configurations, among many others, can no longer be achieved in 1.16 and above. A fuller list of such cases may be needed.
Weird plants[]
Bamboo, kelp, weeping vines and twisting vines could be made to have a "juvenile"-type plant exist somewhere in the middle of the plant throughout 1.16 and its development. This was removed in snapshot 20w27a for 1.16.1.[1]
Redstone configurations[]
Some configurations of redstone wire blocks and its resultant powering were possible with redstone wire up until the changes made to it in 20w18a.