fieldset / legend are not supported by VoiceOver on osX or iOS so I think at least you need to have a warning in https://playbook.cio.gov/designstandards/form-controls/ especially for checkbox, radio or date input.
A possible solution is to use aria-describedby on the input for example :
<fieldset class="usa-fieldset-inputs usa-sans">
<legend class="usa-sr-only" id="legend-id-1">Historical figures 1</legend>
<ul class="usa-unstyled-list">
<li>
<input id="apple-pie" name="apple-pie" value="apple-pie" tabindex="0" checked="" type="checkbox" aria-describedby="legend-id-1">
<label for="apple-pie">Sojourner Truth</label>
</li>
<li>
<input id="key-lime-pie" name="key-lime-pie" value="key-lime-pie" tabindex="0" type="checkbox" aria-describedby="legend-id-1">
<label for="key-lime-pie">Frederick Douglass</label>
</li>
<li>
<input id="peach-pie" name="peach-pie" value="peach-pie" tabindex="0" type="checkbox" aria-describedby="legend-id-1">
<label for="peach-pie">Booker T. Washington</label>
</li>
<li>
<input id="disabled" disabled="" type="checkbox" aria-describedby="legend-id-1">
<label for="disabled">George Washington Carver</label>
</li>
</ul>
</fieldset>
fieldset / legend are not supported by VoiceOver on osX or iOS so I think at least you need to have a warning in https://playbook.cio.gov/designstandards/form-controls/ especially for checkbox, radio or date input.
A possible solution is to use aria-describedby on the input for example :