Rename "accessibility" features to "advanced" features to encourage use?

Accessibility features aren't about encouraging use, they're about being there for the people who need them. A vision impaired user isn't an advanced user, they just need to be able to see your content

The term advanced can significantly vary between domains, I've seen command line programs that use advanced for flags that might damage the system if used wrong, and I've seen advanced settings containing options for exporting/importing settings or generating debug dumps.

“ Web accessibility is about designing web sites, applications, technologies, tools, products and services in an inclusive manner, and thus lifting barriers to communication and interaction that many people face in the physical world.”

https://www.w3.org/mission/accessibility/

For many people these are not advanced features but the basic features that remove or reduce those barriers to use.

I strongly recommend the opposite direction. Do not brand any accessibility feature as something "more" or "additional". Instead you want to weave accessibility features into the baseline of minimum competency such that a failure to consider accessibility is an unforgivable shameful failure of negligence worthy of civil liability and gross financial penalties.

For example, if you drive too fast in a parking lot and accidentally kill multiple people you did not simply happen to miss some advanced consideration. You violently failed a basic fundamental and unforgivable understanding of operating a motor vehicle worthy of spending multiple years in prison and liability of extreme civil penalties. There is plainly evident to the common person without any misunderstanding.

In order to achieve accessibility at Southwest Airlines the leadership mandated accessibility be a common virtue of minimum accepted criteria. This mandate came from corporate leadership in 2014 and violations were unforgivable. WCAG 2.0 AA was the standard and anybody who failed to achieve that minimum standard at every aspect of delivery simply no longer wanted to retain their employment.

I think fewer people who need them would find them.

I suspect people would be a bit confused by the idea that sensible tab key ordering, proper labels, and similar was somehow "advanced".