I've been following the steps on the wiki: don't let a slave boink for 5 days while they're getting aphrodisiacs in their food, then give them a stimulant potion and pass the day. However, it hasn't been working, and the slave I've been giving potions is growing all sorts of mutations now. Have the parameters for acquiring this trait changed? If so, what's the process now?

I'm not sure where you got those steps, but they are not completely accurate. The requirements listed in the traits page of the wiki are mostly correct (30% chance per day instead of 70%); however, the steps you listed do not match it. They must have the slave brand rule for No Masturbation enabled, they must have at least 90 lust, and it must have been at least 5 days since they have had sex. The aphrodisiacs in the food are optional, but a good choice for building lust. The stimulant potion is also optional (it gives 100% chance to gain trait if other conditions met), but it can take a few days if you do not use it.