FHR are not mandatory in "most OECD" nations Paul. Not in the UK either in many catergories.
However, having been in discussion with the Oval Racing and drag racing organisations this week in the UK (neither of whom mandate FHR!) I am tempted to express an alternate viewpoint.
If you are happy to spend thousands on an engine build, or a paint job, why not hundreds on the most expensive single component in the car, the driver? A month off work will cost more than an engine build, a year a whole lot more than the whole car.
A release from FIA in February means that new 5 belt harnesses will not be legal to sell from Feb 2018, yes months ago. 6 point is now the accepted standard. The whole release is available if anyone wants it posted.