Not always as clear cut as that, so many tail pipes are false, decorative bits of shiny chrome that no actual exhaust gases pass through, if you get under the back of some cars you find the exhaust ends in a downward exit and the straight exit(s) you see and feel is/are actually blank and only for image puposes.

Not saying thats the case with your Golf, which sounds like its doing its job well...but never let reality get in the way of wealth transfer into the collander which is the state's finances.

Another one is our 15 year old petrol/lpg Forester, which had a new stainless system some 4 years ago, the tail pipe is as you describe spotlessly clean inside for as far as you can see, doesn't stop us paying £600+ VED penalty mind :-)

I had several turbodiesels as company cars, spanning the pre- and post-DPF eras. My old 2003 Passat PD130 used to have a very sooty tailpipe (no DPF). That was replaced by a 2008 2 litre Mondeo (very clean pipe, had a DPF). Then two Volvos (both very clean).

Our 2015 1.4 petrol Corsa's tailpipe is always spotlessly clean inside.

Which one? The Audi has four, two of which have flaps which only open above, ahem, certain engine speeds and throttle settings. The TR7 has just the one. All five are slightly sooty (I've just been in the garage to finger-wipe). Somehat absurdly both cars, average age 30yrs, are ULEZ entry free.