A web accessibility statement provides your users with information about the accessibility of your content. It shows that you care about accessibility and demonstrates social responsibility.
If you care about Web Accessibility (I know you do) then some of the accessibility related debugging tools in the Chrome browser may come in handy.
For frontend developers the main goal these days is to make sure websites can easily be viewed on a mobile device. If the page layout is not responsive it becomes inaccessible (or at least difficult to access) on a phone or tablet.
But there are other (and often much more serious) accessibility considerations to be taken into account : making sure that web content is also accessible to people with disabilities.
When coding web pages it is easy to forget that not all users will experience them in the same way. Blind people or people with poor eyesight need to rely on screen readers. People who don’t have arms use a stick in their mouth to tab keys on the keyboard to navigate a site. People who are deaf rely on close captions for videos. If we want to make our pages accessible for everybody we need to do a bit more than just adding alt text to our images.