The Accessibility fields Alt Text and Extended Description in the Metadata panel's IPTC tagset were added to LR 13.1, but the implementation is incomplete:
1. In the Library Filter bar and smart collections, they aren't searchable via Any Searchable Field, Searchable Metadata, or Searchable IPTC (like the other fields in the IPTC tagset).
2. In the SDK, the method photo:getFormattedMetadata() doesn't support the keys "altTextAccessibility" and "extDescrAccessibility".
3. The Lightroom Classic Programmers Guide should document the tagset field ids "com.adobe.altTextAccessibility" and "com.adobe.extDescrAccessibility", in two sections:
Token Patterns (page 62)
Custom Metadata Tagset Example (page 77)