Recent additions to MPEG-DASH specification (ISO/IEC 23009-1 5th ed., AMD3; also SCTE 214-6) offer new ways to implement low-latency low-delay playback: using so called Segment Sequence Representations (SSR).
MPGE-DASH manifests advertise this flavor of data by including a respective EssentialProperty.
Before Shaka Player Demo added support for this technology, they ignored the respective adaptation set as “unknown” and “unsupported”, which seems to be the right way to address something you do not understand.
Here is the Microsoft’s approach.
They do support MPEG-DASH playback in, for example, their XAML MediaElement (below is the screenshot from PlayReady sample):
They apparently support adaptation set switching urn:mpeg:dash:adaptation-set-switching:2016 as they mix downloads from regular and SSR-enabled adaptation sets.
However they do not support segment sequence representations urn:mpeg:dash:ssr:2023 themselves as they attempt to download content without replacing $SubNumber$ placeholder, and, hence, they ignore the EssentialProperty markup: the adaptation set is marked as something new they do not understand but they take chances to play it back.