Based on the format for required date searches, it appears that US Core requires servers to support date searching without prefixes, for example

GET [base]/Procedure?patient={Type/}[id]&date={gt|lt|ge|le}[date]{&date={gt|lt|ge|le}[date]&...}

where {} indicate optional parameters.

The details in the search parameters themselves show support for the "eq" prefix as a MAY. Since searches without a prefix are equivalent to searches with "eq", this can make it seem that support for search without a prefix is not required.

Language should be added to make it more clear that servers are required to support date searches without a prefix.

Zulip thread: https://chat.fhir.org/#narrow/stream/179175-argonaut/topic/US.20Core.20date.20search.20with.20comparator