I come with an update, friends!
Sephiroth2000 and _Sephiroth_ have either changed their names or left. The other 20 Sephiroths that are listed on that post by Qhimm are still around (though not necessarily active).
In addition, there are a few more:
Sephir[o]th
Sephirosu
sephiroth266
Sephiroth#pt#
SephirothClone
Bizzaro-Sephiroth
sephirothf
MesiahSephiroth
Sephicloud
SephyCloudyWhatserface_Lil.Yufi
darthseph23 (my personal fave)
sephi69
SephyX
E-Sephie
sephblade
EmoSephy
sephy_forever19 (and amen to that)
So there are 37 variations on Sephiroth, not including Sephiroth2000 and _Sephiroth_.