I can confirm it's still the case in 20w12a as well.
BisUmTo your command does put the border visuals in the nether to that area, but that also moves the worldborder in the overworld, which is not always what people want. Considering the coordinate system in the nether, one might want to limit the worldborder size in the nether to 1/8th the size of the overworld. I had one of my friends die on my private server due to also being able to walk over the worldborder due to there being no visual clue, which is how this bug was brought to my attention in the first place.
Basically the problem afaik is that no matter what dimension you're in, the visuals of the worldborder seem to match those of the overworld, but the actual worldborder can be set per dimension individually, which means there can be a mismatch between the visuals and the actual deadly area. If the overworld has the smallest worldborder, it'll only be a minor issue. If you have it the other way round however (like I did until I learnt about this bug and will again if this bug gets fixed), players have no idea that they're past the worldborder and will start taking damage, seemingly for no reason.
I can confirm it's still the case in 20w12a as well.
BisUmTo your command does put the border visuals in the nether to that area, but that also moves the worldborder in the overworld, which is not always what people want. Considering the coordinate system in the nether, one might want to limit the worldborder size in the nether to 1/8th the size of the overworld. I had one of my friends die on my private server due to also being able to walk over the worldborder due to there being no visual clue, which is how this bug was brought to my attention in the first place.
Basically the problem afaik is that no matter what dimension you're in, the visuals of the worldborder seem to match those of the overworld, but the actual worldborder can be set per dimension individually, which means there can be a mismatch between the visuals and the actual deadly area. If the overworld has the smallest worldborder, it'll only be a minor issue. If you have it the other way round however (like I did until I learnt about this bug and will again if this bug gets fixed), players have no idea that they're past the worldborder and will start taking damage, seemingly for no reason.