Every airline has its own contract of carriage. This is the list of legal responsibilities and rights between the air carrier and their passengers. And when you buy a ticket from an airline, you agree to that very same contract of carriage. It's just like the terms of service of a website. You abide by their rules if you choose to use their service. While there are inevitably similarities between these contracts, each airline has their own terms that your should be aware of.
As of January 22, 2025, Spirit Airlines instituted new additions to their own contract of carriage which are related to passenger appearance. Tattoos that are considered obscene or offensive in some fashion could be an issue. The same is true of any clothing that could be deemed lewd or revealing, or does an inadequate job of covering the body. That includes bare feet. It's worth nothing that some of these rules were already in place. The terms have just been upgraded and clarified.
That being said, the "clarification" is still in the favor of the airline and not the passenger. Gate crew and plane crew can determine what does and doesn't break the contract of carriage based on their own sensibilities. Infringing on the contract of carriage can result in a passenger being prevented from boarding or being removed from the plane. It's not common, but it has happened. If someone at the gate doesn't like your tattoos, you could conceivably be barred from boarding.
Based out of Florida, Spirit Airlines is one of the top discount air carriers in North America. It offers flights across the United States as well as to the Caribbean and Latin America. The airline has been no stranger to controversy and customer complaints over the years. Towards the end of 2024, a flight into Port-au-Prince, Haiti was hit by gunfire and had to be rediverted to the Dominican Republic.