Really commonplace, alas. Did a quick search for Transit DPF (just to see about later transit issues), and one of the first results was this company:

https://www.avontuning.co.uk/services/dpf-removal-delete

Proudly professing their removal service, and with a caveat at the end to say that it makes the vehicle illegal for road use and is for off-road use only.

I don't think it should be allowed, personally, but I'm sure there are libertarians who think they have a right to pollute...