Yes, a natural liquid fabric softener does exist. Simple solutions and recipes are usually the ones that are most often overlooked, and usually the ones that are most worthy of being shared. This simple fabric softener solution only requires one ingredient: vinegar!
Add the vinegar to the washing machine during the rinse cycle, to the fabric softener dispenser, or a fabric softener ball (like a downy ball) at the beginning of the wash cycle.
Exercise Clothes: The acid in vinegar breaks down elastics over time. Don't use vinegar with elastic fabrics, like your favorite Lululemon yoga pants. Instead, opt for the non-vinegar options in the post above, like wool dryer balls, salt, or homemade dryer sheets.
Castile Soap: Yes, you can use this vinegar solution with a castile soap based laundry soap. You will not have an issue since you're not mixing the two ingredients directly but instead using them in a giant washing machine (filled with lots of water).
Essential Oils: Some people like to mix a couple of drops of essential oils with vinegar in a small bowl before adding to the wash, but I don't see the point in doing this. The clothes will not end up smelling like the essential oil. Instead, save the essential oils for other natural cleaners.