I was wondering how such a ledger might work, given that it should be about as difficult to quantify this kind of debt as it may be to estimate the time and effort required to implement certain changes in general. Later on in the post, there's this nugget of gold:

    So how would this ledger work? Well, for starters it has to
    track what we can’t do because of current technical debt.
    It should also be updatable to note any complications to 
    subsequent work or things you can’t do yet because for old 
    design decisions. At this point, you’re tracking the 
    “principle” (the original design decision causing technical 
    debt) and interest (the future work that was impacted by 
    the debt).

That's it, isn't it? You need to define the principle and the interest, and these two are actual tangible things in the form of specific decisions (principle) and things that are now adversely affected by them (interest.) If these are linkable, then it should become trivial to compute a number to this debt, whether it is simply just the number of things adversely affected or some other kind of aggregate like the combined estimated effort of those adversely affected things. This debt could probably be calculated in many different ways, but the fact that you can properly quantify it should make decisions on whether to tackle or ignore the debt much more informed.

This was an eye opener for me – thanks for sharing!

Figuring how to link the “principal” and “interest” is definitely the lynchpin ... but if you’re not even noting there is principal and interest, you’ll never be able to recover