Riding any motorcycle without proper support for your back can potentially lead to discomfort or strain, regardless of the specific model or brand. The Harley-Davidson Softail, like other motorcycles, is designed to provide a comfortable riding experience when proper ergonomics and riding posture are maintained.
Using the backrest or back support provided on a Softail can help alleviate some of the stress on your back by providing additional support and stability. However, whether it is "bad" for your back to ride without sitting on the backrest ultimately depends on your individual body and riding preferences.
If you choose to ride without using the backrest, it's important to ensure that you maintain a good riding posture, which includes keeping your back straight and engaging your core muscles to provide stability and support. Additionally, taking breaks during longer rides and practicing good stretching exercises can help reduce the strain on your back and enhance your overall comfort while riding.
If you have any specific concerns about your back or any pre-existing back conditions, it's always advisable to consult with a healthcare professional or ergonomic specialist who can provide personalized advice based on your individual circumstances.