Not necessarily true. You might be in violation of their sales license, but that is a legal issue about sales licenses. I'm not sure of the company's power to dictate the use of their product after the original owner purchases it.
Because they have recognized the importance of shoulder harnesses to safety, the FAA changed the policy on the upgrades of shoulder harnesses. A shoulder harness system can be installed in the OP's airplane (pre 1986, no shoulder harness) without a STC via FSDO field approval on a 337. If a particular system was already approved for installation in the type via STC and requires no engineering, this might even be installable as a minor change.
https://www.faa.gov/aircraft/gen_av/harness_kits/media/shoulderpolicy.pdf
The description of the process for field approval is on page 12 or as a minor change on page 14.
There's even mention in this document of automobile 5 point racing harnesses and how to install them. That how important the FAA considers safety harnesses.