Minecraft wiki entry for Monolith, associated text: By setting "Biome Scale Weight" to negative values in old customized worlds, they could generate  from snapshot 14w17a for 1.8 to snapshot 18w05a for 1.13, but with the removal of the "Customized" world type altogether in snapshot 18w06a for 1.13, this can no longer be recreated.
Monoliths can once again be generated in 1.16+ using customized worlds, however the settings to actually produce them are unknown.
It is not known if the reworked terrain generation in 1.18 is capable of harboring monoliths.
In between Java Edition Infdev 20100624 and Java Edition Infdev 20100629 the bedrock layer directly underneath the monolith was raised to sea level, making the void easily accessible underneath the monolith.