While new companies might not prioritize web accessibility simply because of price, it can be seen as a great insurance policy. Even if you don’t fall into one of the categories mentioned above, if you want to prepare for the future and ensure no lawsuits, becoming web accessible is the way to go. Not to mention being available to persons with disabilities is very important and says that you care.
The Federal Communications Commission (FCC) enforces regulations covering telecommunication services. Title IV of the ADA covers telephone and television access for people with hearing and speech disabilities. It requires telephone and Internet companies to provide a nationwide system of telecommunications relay services that allow people with hearing and speech disabilities to communicate over the telephone.
Two agencies within the U.S. Department of Labor enforce parts of the ADA. The Office of Federal Contract Compliance Programs (OFCCP) has coordinating authority under the employment-related provisions of the ADA. The Civil Rights Center (CRC) is responsible for enforcing Title II of the ADA as it applies to the labor- and workforce-related practices of state and local governments and other public entities. Visit the Laws & Regulations subtopic for specific information on these provisions.

In an August 2016 case involving the University of California Berkeley, the DOJ ruled that the public university was in violation of ADA Title II (similar to Title III but it instead applies to government organizations) because their YouTube channel’s videos didn’t include captions for hearing impaired visitors. The DOJ found this to violate the ADA as deaf users did not have equal access to the online content.

In short, the ADA is meant to protect disabled individuals as they go about their daily lives. These regulations ensure that people with disabilities are not denied entry into the above places or denied services by a company due to their disability. It is important to note that these regulations are now applicable to services that are provided online or through other digital formats. For example, if a company accepts job applications online, then it must ensure that a person with a disability can also apply for the job online. In other words, it is illegal to have barriers on the website that would keep the disabled individual from successfully completing their application. Fortunately, the ADA guidelines help to remove barriers and ensure that the Internet remains a space that people of all backgrounds and disabilities can use.
These rules nearly made their way to the Supreme Court in 2019. In 2016 a web accessibility civil suit was filed against Dominos Pizza. In the following years, the case filtered through layers of judgements and appeals, and Dominos eventually appealed to the Supreme Court. Unfortunately, the Supreme Court declined the case and returned it to a lower court. Though this provides no final clarification, the ruling by a lower court in favor of the plaintiff supports the legal requirements for web accessibility.
The need to make websites, mobile apps, and other online properties accessible to all is only going to increase as time moves on. Smart business owners will do well to get in front of this issue and make sure that their websites are ADA compliant now so that all their customers have the equal access to the resources they offer. Not just because they want to avoid a lawsuit or government action, but because it’s the right thing to do.
You are correct, ADA defines an “employer” as any person who is 1) engaged in an industry affecting commerce; 2) employes 15 or more full-time employees each work day; and 3) for at least 20 or more calendar weeks in the year. In the context of physical spaces, ADA would not apply to companies with fewer than 15 employees. However, courts don’t seem to have come to a consensus on what digital compliance really should look like. Because websites can be accessed anywhere in the country, small business owners might potentially face lawsuits in unfavorable jurisdictions. If you are a small business owner, I would recommend consulting with an accessibility lawyer and ask what they recommend.
Yes, in time non-ADA compliant websites will be penalized in the search engines. It's already on their radar. But for now they are still negatively affected, just in an indirect way. Live traffic behavior (user experience) is now factored into Google's algorithm and is a ranking signal. Negative user interactions absolutely have negative affects on rankings.
The ADA statute identifies who is a person with a disability, who has obligations under the ADA, general non-discrimination requirements and other basic obligations. It delegates fleshing out those obligations to federal agencies. The agencies issue regulations and design standards. The regulations have the details on the rights of people with disabilities and responsibilities of employers, state and local governments, transportation providers, businesses and non-profit organizations. The design standards specify how many entrances need to be accessible, how many toilet rooms and the design for those elements.  To know what the ADA requires, you need to read the law, regulations and design standards.  
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