how are these not already prohibited? Did they slip through the cracks or something?
From the Senate debate:
This bill amplifies the IRL 8th Amendment (whatever number equivalent in Atlasia) by specifically enumerating certain types of illegal torture. The bill contains language to ensure that this is not presented as an exhaustive list and that judicial relief can still be available for such unlisted torture methods via the constitutional ban on cruel and unusual punishment.
Are any of these currently in use or upheld by legal precedent?
There really arent cases on a lot of these. Some punishments like castration and purposeful scaring were common in the US when the 8th Amendment was ratified, as was gibbeting, branding, and whipping. Coerced sterilization and abortion happened in prisons in the 20th century, as did lobotomies. Many of these could become issues with an evil government.