Looking at the absent-reason code system below, OTH (other) is not there whereas it is supported in HL7 v3 nullflavor. Is the use of “OTH” as absent-reason discouraged in FHIR?
If you have codes that aren’t covered by the data-absent-reason extension, define a different extension that communicates what you need. There’s no need for an “other” concept. (The only reason to say data is unavailable is if you have something useful to say about that - otherwise you could just omit the element.)
Well, it’s not there now and I can’t think of why we’d add it. If you have a use-case, you can always submit a change request arguing why that code - or any other - should be added.