The agencies are more challenged by the needs to create an accessible frontend then an accessible backend. The latter is obviously not reasonable for a single client.
Sidenote: it will help to answer general questions about a11y in the Slack a11y channel. So far the a11y team is single-mindedly focussed on the BE. Actually work on anything besides the new page module.
How would the Q&A differ from the efforts mentioned here Accessibility best practices guide for TYPO3 ?