Sorry, but you're right to be concerned and in my opinion this is outrageous.
Let me explain - In the early 2000s these loan schemes were being pushed by accountants as a way of saving tax because the half-thought-out IR35 rules made everyone panic. The important thing about this is that AT THAT TIME this was all legal. These schemes could even be registered with HMRC - who did nothing. You might have a view that it's immoral, but the important thing is that it was legal. Move forward to 2016 and the HMRC have now decided that this wasn't legal after all. They've jumped in their Back to the Future time machine and changed the law retrospectively.
It's a bit like the government changing the speed limit on your street from 30mph to 20mph overnight, and then sending you tickets for all the times in the past you drove at 25.
Now the view of these schemes has changed - I think most people recognise it's not ok anymore - but this is hardly 'fair' as Mr Hammond might say. The thing to remember is, they're looking at any loans since 1999 and they're going to be treating ALL the money paid out under those loans (if they are still outstanding) as income. This will be added to your 2018-19 tax return as one lump sum of income that will push many people into the maximum tax band.
Believe me, a lot of people don't know this is coming and their circumstances may have changed dramatically in the last 20-odd years, they may be retired, not working, anything. The taxman doesn't care - he's going to treat that money you had as income, add your NI onto it, and interest too - and just in case you don't feel cr*pped on enough you'll have until Jan 2020 to pay.
This is going to wipe out a lot of people, and there isn't anything you can really do about it apart from prepare for the worst. About the only way to avoid this is to jump under a bus. Sadly I can see people taking that option - Fairness. Don't make me laugh.
I feel so sorry for those not able to pay, but what's just as worrying is the precedent - if the taxman decides he can turn back time when it comes to this then where does it end - your pension? your ISA? Don't bet against it.