The technology used to track your heart rate really hasn't changed much. This article will explain how the Heart Rate sensor works: Your heart rate. What it means, and where on Apple Watch you’ll find it. - Apple Support
Essentially, the Heart Rate sensor uses green light to read your heart rate. Tattoos distort that green light and therefore prevent your heart rate from being read accurately.
This article says the following:
Permanent or temporary changes to your skin, such as some tattoos, can also impact heart rate sensor performance. The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings.