The STU3 version of CapabilityStatement included an element acceptUnkown
This was removed from the R4 version (see FHIR-14647)

I believe it is still a valid requirement to declare whether a server will accept unknown extensions, or not. Separating this out to be a more explicit element ( say *acceptUnknownExtensions *) as a boolean would simplify this.

The definition within a "requirements" capabilityStatement would be clear.

Definition within an instance would need to be declarative against stated behaviour. This would need to be decided upon, but broadly speaking would need to be one of

• If an unknown extension is received and there is no support then return an error
• if an unknown extension is received and there is no support then "drop" the extension

This would not apply to "Modifier Extensions" which already have clear guidance within the standard.