I asked HMRC what open source software there was for "Making Tax Digital". Their answer: use their software-finding tool. I'd asked them the question because their tool doesn't make that information available.

Does such a thing exist?

It seems the whole push behind "MTD" is towards real-time awareness of everything. And the plan is for that to be done by us all renting* software from random companies. Companies who'll probably sell/use your financial information (or leak it or be hacked).

*Sure they claim there are "free" options although many of those listed in their tool can't actually do all the things HMRC already require and HMRC are careful to tell you it is your fault for which software you pick (you have to pick).

No doubt there are plenty of people who'll insist that HMRC shouldn't be making software. But if they legally mandate you have to use software I think they should make something open source themselves. That does the minimum they require.
Anyway, this is stronger encouragement for me not to be self-employed in the future.
@slowe Which seems to have been one of HMRC's driving objectives for the last 30 years.
@slowe yeah; I closed my long-dormant company rather than deal with this