Due to an old bug in Alpha versions of the game and a feature in the new snapshot. Alpha worlds generate caves underneath them with thousands of holes in them.
Alpha worlds had an issue where at Y=0, stone would generate randomly throughout the bedrock causing holes to the void. While this was fixed for new generation, this bug has remained in worlds generated beforehand.
In snapshot 21w43a and 21w44a, along with the new feature to generate stone and caves under old worlds, another feature was added to maintain farms and worlds that utilized access to the void so that nothing would generate under blocks other than bedrock. This mixed with the previously mentioned Alpha bug has created the problem encountered.
Keep in mind, the only blocks that commonly find their way to Y=0 throughout almost all versions is typically stone, lava, and water. This feature to maintain farms and worlds can still be implemented if blocks like stone, lava, and water are changed to work the way that bedrock does to allow generation underneath them.
MC-240030 Holes in worlds created in Alpha and Infdev below y=0 after conversion