API 6D doesn't exclude threaded valves specifically but the text addresses flanged valves or weld-end valves without mentioning npt
The big loophole is on page 34 in paragraph 6.7.3 where the standard says:" Other end connections such as special flanges or mechanical joints may be specified by the purchaser"
Other mentions of "threaded" in API 6D include 6.9 pertaining to bypass, drain and vent connetions and plug entries to be drilled and threaded; and the associated table 7.
Note that 6.1 and 7.1 reference ASME B16.34 (or an equivalent standard by agrement).
ASME B16.34 excludes threaded and socket weld valves larger than NPS 2 1/2.
Avoid any confusion - don't specify threaded valves for process service as the first block. You can see that exceptions may exist in small sizes that accommodate vent and drain valves through 2 1/2 NPS.
In instrumentation we use threaded instrument valves downstream of a line class block valve. However, I only buy flanged line-class valves like shutdown valves and control valves for hydrocarbon applications. API 6D is a common requirement for oil and gas applications. ASME B16.34 is another widely used valve requirement. Obtain copies of these standards.
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