When some humans, humanoids and mythical creatures have an incredibly powerful magical regeneration it seems quite obvious that in times of need and growth some countries would try to capture them and use them as ''basic resources''.
This is not really a spoiler because it's on the first 2 pages of the book but one example is Fire Punch, in order to survive a kid is using his magical regeneration to create food and burning fuel by chopping his limbs continuously.
One real world example is Culling, basically creatures bred and tortured for the only purpose of creating easily accessible resources, whether such creature dies or lives is based on their ability to produce such resources and being spared and surviving might actually be considered a destiny worse than death.
I believe it is historically accurate to assume many kings, queens and people of power would have indeed enslaved such ''people'' and used them for things likes experimentation, food or flesh-shields ... or for pretty much anything actually ...
There are different means of getting the magical regenerative abilities, some of them are either training to become a magical healer or simply being born in a magical forest or being born from parents who had this ability, so it is quite common and they are spread across a world with many far apart isles, almost like if our world had been broken into small pieces distanced by water. Technology is mostly pre-victorian and a few empires and nation have powerful artifacts created with a mixture of engineering and magic, those artifacts can be used to create portals, transform lead into gold and other things... but they are incredibly rare. The rest of the world is made by mostly tribes and villages.
What is the best way to protect self-healing creatures from being turned into slaves?
Religion is excluded because in my setting deities do actually exist but everyone knows they have no say on the subject. A powerful cleric could simply ask a god or goddess if she or he have anything against it and they would respond that they don't care.
Yes, the energy and materials needed for regeneration are infinite and come from magic.