Aloha Tatiana! I wish your questions had straightforward easy answers – and if you are government funded it kind of is. (See here: https://www.ada.gov/websites2_prnt.pdf) if you are not government funded, this is something that is currently being debated in the legal system. That being said, the WCAG 2.0 guidelines are a good set of guidelines to help protect yourself if you feel you should. (You can see more on WCAG here: https://www.yokoco.com/find-out-how-to-make-your-website-compliant/)
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.
Various courts around America have ruled that commercial websites are places of public accommodation and thus subject to ADA rules. Other cases have concluded that websites are bound by ADA regulations if there is a close “nexus” between the site and a physical location, the most famous example being the ruling against the Winn-Dixie supermarket chain for not making its site accessible to users with low vision. Other courts have decided that the ADA as written simply does not offer any protections for online users. With no overarching federal rules in place, it’s difficult to make a definitive statement about whether or not any given website is governed by ADA accessibility rules.
The ADA is the Americans with Disabilities Act, passed almost 30 years ago in 1990. This act was originally put in motion pertaining solely to physical location. It requires establishments to provide people with disabilities easy access to various levels throughout the business. This act was set to achieve an equal experience to all people, handicapped or not.