Currently, there is a safe harbor clause that allows your existing content to remain as it is, unless altered after January 18, 2018. However, the guidelines do pertain to any page that has been updated after that date. So if you want to avoid the legal costs of being found non-compliant with the ADA, it’s best to make the necessary changes to your website now.
I am struggling with this right now. Alt text, captions, audio (read to me) options are good. I work with an accounting software app that redesigned its palette to red, black, and gray. No green to signify income. Made no sense to me and their gray text (also very skinny sans serif) made reading many nav bars difficult. Some URLs are accessible from the build-out, but the DIY sites often don't have the same capacity. As with secure sites, I think google will be downgrading those sites with little or no compliance or accessibility add-ons.
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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.
Every year numerous lawsuits are taken against businesses that fail to follow the ADA’s proposed requirements for web accessibility. This failure occurs because organizations, including state and local government entities, fail to read the ADA Best Practices Tool Kit.8 They also do not follow the most up to date version of the WCAG.9 These two failures are not only detrimental to people with disabilities who want to effectively browse the web, but they are also inexcusable in today’s digitally driven world.
When the law was enacted in 1990, it did not specifically address website accessibility for the disabled, but this has become a much-discussed topic in recent years. In 2006, Target settled a class action lawsuit alleging was inaccessible to the blind, in violation of the ADA, and in 2015 both Reebok and the NBA were hit with a class action lawsuit that alleged their websites did not accomodate the blind and visually impaired.

The gray area is a matter of scale and purpose. If you have a small business which serves as specific market it's a good idea to make the site as user and disability friendly as possible. If, however, your market serves segments with a likeliness to have various handicaps or disabilities you should address that with an ADA Compliance review of the site. Also consider the scale and purpose of an organization. Publicly traded large businesses need to concern themselves more than a single location family owned retailer and if someone sues your flower shop because the text was too small or didn't have "image alt tags" causing them distress... That's pretty frivolous and would likely not stand.
Over 10,ooo business owners were sued this year (2019) and they expect that # to increase to over 100,000 in 2020.  And these are no-joke lawsuits.  They settled for $20k - $6million, average was $50k...crippling to most small business owners, and you can't win these cases.  Unlike GDPR, the ADA law doesn't require any notice before a lawsuit can be filed for non-compliance, and it doesn't provide any time for a company to take positive or corrective action before its filed, so there's no real defense.  It's basically just a race to see who wins...the site owner that protects his or herself, or the dirty lawyer that finds them before they did.  Getting sued is a REAL risk, not the usual fake scare we usually see that disappears over time.
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.