I did a search on PubMed for 'treatment modality' and got 413 results, 66 of which were reviews. Here's some of the titles (just taken from the first few results):
- Analysis of nocturia with 24-h urine volume, nocturnal urine volume, nocturnal bladder capacity and length of sleep duration: concept for effective treatment modality.
- Acupuncture as a treatment modality for the management of cancer pain: the state of the science.
- THE IMPACT OF TREATMENT MODALITY ON INFECTION-RELATED HOSPITALIZATION RATES IN PERITONEAL DIALYSIS AND HEMODIALYSIS PATIENTS.
- Autologous stem cell transplantation as treatment modality in a patient with relapsed pancreatoblastoma.
- Dysphagia, stricture, and pneumonia in head and neck cancer patients: does treatment modality matter?
- Somatostatin receptor scintigraphy as a potential diagnostic and treatment modality for thyroid follicular-cell-derived cancers.
- Silver: an age-old treatment modality in modern times.
- Dendritic cell vaccination as a treatment modality for mesothelioma.
- Extensive basal cell carcinoma of the forehead and anterior scalp: use of helical tomotherapy as a radiotherapy treatment modality.
- New treatment modality for maxillary hypoplasia in cleft patients. Protraction facemask with miniplate anchorage.
413 results out of the tens of thousands of journal articles indexed by PubMed is relatively few. I'm sure we could probably at least double that number by searching on different phrases that use the word but that's still not a very large number of results.
So it does look like the term is used, but not very often. It's probably an archaic term that has coopted by alt-med promoters.