Man pages matching name tsmimehdrfieldlengthget in section 3ts:

TSMimeHdrFieldLengthGet.3ts.en	debian 10 / trafficserver-dev
TSMimeHdrFieldLengthGet.3ts.en	debian 11 / trafficserver-dev
TSMimeHdrFieldLengthGet.3ts.en	debian 12 / trafficserver-dev