The inert subtrees portion of the spec says:

When a node is inert:

• Hit-testing must act as if the 'pointer-events' CSS property were set to 'none'.
• Text selection functionality must act as if the 'user-select' CSS property were set to 'none'.
• If it is editable, the node behaves as if it were non-editable.
• The user agent may ignore the node for the purposes of find-in-page.

…but it does not acknowledge that inert subtrees should be hidden from accessibility APIs and assistive technology. To my knowledge, this understanding is shared by the accessibility teams of every major vendor, so I was surprised to not see it explicitly mentioned in the spec. If there is disagreement on this point, I'm unaware of it.

The inert explainer introduction makes it clear that:

If an Element is inert, it is completely non-interactive: it isn't targeted for any user interaction events, and it is removed from the focus order, and it is not exposed to assistive technology.

If there is no disagreement, this shared understanding should be codified with another bullet such as:

• The user agent must not expose the node to accessibility APIs or assistive technology.