On 2b2t, several water source blocks were placed in the Nether despite the ability to do so being illegal. Normally in survival Minecraft, placing a water source block in the Nether will instantly evaporate the water. Various players such as popbob and policemike55 were able to leave illegal water anyway and some of it lasted for almost an entire decade.
According to a reddit comment by the oldfag cgtdream:
There used to be water in the nether, mostly dotted around nether spawn, and below 1k (and in other areas as well)...However, they started to disappear for various reasons after being able to place water in the nether was patched out, and were pretty much all "gone" after the rusher and youtuber era. There are ways to have flowing water in the nether now (outside of commands), that involve redstone, but they arent pratical for anything other than "ooo wow!"
According to _Courier6_, several water source blocks in the nether still exist on the server, including water that was (probably) placed by popbob which was toured by SalC1 who made a popular video about it.
It is unknown exactly how the source blocks were placed, as back in older versions of Minecraft, players were able to place ice in the Nether, melt it and then it would become water without evaporating. But at the time, it was impossible to obtain ice in survival Minecraft. By the time ice blocks could be mined by using silk touch pickaxes, Mojang had already patched ice melting in the nether. Only 1 version of Minecraft exists where players could collect ice and place it in the nether for it to melt, which was 1.9 Beta PreRelease 4. It was never officially released, and as such 2b2t was never updated to that version. The most likely explanation is the usage of backdoors that allowed players to obtain water source blocks (which no longer exist in the game) in their inventories. Using analysis of the terrain generation such as lava pockets, Nether Fortresses, chunk trails, and their proximity to spawn, players have determined that all of the known water source blocks were placed between versions 1.0 and 1.4. Additionally, it is theoretically possible for them to have been placed as late as version 1.7.10, making all source blocks pre 2016.
Water source blocks are a very rare on 2b2t, as very few were around to begin with. Much of the water was griefed many years ago according to cgtdream. It is assumed that much of the locations of the existing water are unknown. Usually, when a player discovers Nether water, they try to keep the location secretive to prevent griefing. Some water was placed above the nether roof, such as the one at Wassrige Holle. These were all destroyed in late 2020, when an update to the server plugins automatically erased all blocks above the nether roof. In 2020, Armorsmith griefed the Nether water which was placed by policemike55, making it the first documented griefing of Nether water on YouTube.