Kris Rivenburgh differentiates plugins from overlays and explains that while plugins can provide benefit, they can't make a website "ADA compliant" or WCAG conformant.
Website accessibility plugins can serve multiple purposes, from general, marginally beneficial accessibility options (e.g., increase text size, change color scheme, etc.) to specific modifications such as adding form field labels in specific instances. Wordpress and Shopify plugins are available, but note that some plugins are merely a ruse (i.e., an overlay under a different name).
Legitimate plugins will disclaim their limitations and not claim "full compliance" or WCAG 2.1 AA conformance.
Kris offers accessibility services including WCAG 2.1 AA manual audits and remediation at https://accessible.org.
Kris also wrote the book on ADA compliance for digital assets. You can find out more about The ADA Book at https://ADABook.com.
Transcript:
https://adabook.com/accessibility-plugins-website-ada-compliant/