Are there international standards relating to cruelty in the production of brushes?
No, there is no such ISO standard. Many countries regulate some aspects of animal treatment. These are generally effective at making the predominant kinds of natural bristles cruelty-free, but largely ineffective at addressing the issue this question focuses on.
Clarifying the nature and source of the problem
The question and comments refer to overseas fur farms where animals are mistreated and the industry has poor standards. This misstates the nature and source of the problem, and suggests that one could buy cruelty-free sable brushes from caring, well-regulated American fur farms if only there was an identified standard printed on the box of brushes. So let me start with some context.
Natural brush bristles come from two kinds of sources. The vast majority come from domesticated farm animals, like pigs, horses, ponies, oxen, goats, etc. The animals are raised for other purposes and the hair/bristles are a minor by-product. Domesticated animals are a big investment, intended to provide service over a long time or mature to become food. As such, they are generally well-treated.
Harvesting hair from these live animals entails processes that most people would not define as inhumane; anything traumatic to the animal would be detrimental to the purpose for which they are being raised. Hog bristles are the predominant source of natural brush bristles, and they are recovered from the skin as a waste product after the animal is slaughtered for food. For animals raised for food, at least developed countries typically have some regulations on their humane care and slaughtering.
So the vast majority of natural brush bristles would be deemed cruelty-free by most people who don't have issues with the domestication of farm animals, and that is not the source that is the focus of the question.
The other kind of source is non-domesticated animals, which accounts for a tiny portion of natural bristles, but is where most questions of animal cruelty come in.
Each type of animal hair has some unique characteristics. There is a wide range of domesticated animals, from which a variety of hair characteristics are available. Some non-domesticated animals have hair with other characteristics, which make good brushes.
Some of these animals can be bred in captivity, and some don't do well even with that, requiring that they be captured in the wild (or they are easily trapped so it isn't worth the investment of raising them in captivity). For the most part, their fur is the reason they are raised or captured. Often, the body portion of the pelt is used for fur and the tail is used for brushes.
For the animals bred and raised in captivity, the money is in the fur, so there is an incentive to keep the animals healthy and injury-free. But the fur farms are not animal sanctuaries where caretakers try to provide the best possible quality of life. The goal is to keep the fur uninjured and reasonably healthy only for as long as it takes to grow to mature size. The non-domesticated animals bred and raised in captivity are not like pets or farm animals. They're wild, fast, predatory animals that are contained in an enclosure. Their care is different from raising domesticated herbivore food animals with a similar goal.
Animals trapped in the wild are typically killed at that location as soon as they are discovered, so there's no incentive to use non-injuring traps, which would greatly reduce productivity. Trapping is often performed by people on a subsistence income who view the pelts as a commodity; humane treatment for the "varmints" is typically not a consideration.
Various methods are used to kill the animals bred and raised in captivity. The incentive is to do it in a way that doesn't ruin the fur. A humane method that doesn't induce panic would be consistent with that, but there's no way to know what method was used. For wild-captured animals, the method used to kill the animal at the capture location typically does not guarantee an immediate and painless death, although an experienced trapper can quickly put the traumitized, trapped animal out of its misery.
It's not impossible to make brushes that most people would consider cruelty-free from non-domesticated animals raised in captivity, but we don't know which sources accomplish that. For wild-caught animals, it's likely that they were not trapped and killed by methods most people would consider cruelty-free, and we don't know what brushes contain their hair.
Brush labelling as to hair content is very unreliable; the labelled species is often merely suggestive of the brush's characteristics; it may or may not contain hair from the labelled species, or may be a mix of different animal hairs. As a purchaser, it is impossible to reliably know whether a specific brush or brand qualifies as cruelty-free. The default assumption should be that it probably is not.
What can you do?
Brushes made from the hair of non-domesticated animals exist because the hair has characteristics that are different from the hair available from domesticated animals, and some people are willing to pay the cost of obtaining hair from those sources. It is available for a price, but you can't count on it being cruelty-free.
Some brush manufacturers try to emulate the characteristics of these animal hairs by combining hairs from different domesticated animals and/or synthetics. You can try those to see if they meet your needs. But if only the "real thing" produces the results you want, realistically, you would probably need to weigh use of the brushes against your priority that they be cruelty-free.