Your question contains several incorrect premises, hence before providing an answer I will first correct those misconceptions as any discussion based on incorrect premises cannot be very productive.
Correction of Misconceptions
Ten years ago, negative interest rates were an impossibility, an intellectual curiosity.
This is not true at all. Clearly empirical research shows that there were periods when interest rates in some countries were slightly negative. For example, Kugler, P., & Rich, G. (2001) show that Switzerland had slightly negative interest rate in late 70s. That is about half a century ago and the paper itself is 20 years old and this was well known. So your statement is just factually not true.
In fact negative interest rates and possibility of thereof enjoyed periodic waves of interest, the most early ones dating all the way back to early 20th century (See Ilgmann, C., & Menner, M. (2011) and sources cited therein).
Just read a claim that there is still an absolute lower bound, even if it happens to be a little bit below zero. But that's premised on an inflationary world.
One cannot prove a negative but I study macroeconomics for quite some time and I have never seen anyone saying that zero lower bound (ZLB) or even effective lower bound (ELB) are premised on inflationary world. I would like to be corrected if anyone economist ever made such claim, but I doubt it and even if some do this is definitely not any claim that would be generally held by mainstream macroeconomists.
And in an economy that is deflating at -.06, is -.05 still still the 'real world lower bound' of interest rates? Can a lower bound be said to meaningfully exist, or is there only the spread between the rate of deflation and the interest rate?
ZLB or even ELB does not depend on deflation or any arbitrary rate of deflation. ZLB or ELB exists independently of what the rate of inflation/deflation is in the economy. ZLB or ELB as pointed by @BrianRomanchuk in his excellent +1 comment refer to the bounds on nominal interest rates, and even more specifically nominal interest rates on deposit accounts. Real interest, or even other nominal interest rates (such as rates implied by ECB's long-term repo operations (LTROS) rates see The Economist for explainer) can be arbitrarily negative.
The reason why ZLB or ELB exist is the existence of cash. Due to existence of cash - which is equivalent to financial paper with implicit zero nominal interest rate - there must be some ELB. Since cash and money on deposit accounts are theoretically perfectly fungible the limit on nominal interest rates should be zero. Of course in real life there are issues such as storage costs. Even without negative interest rates you are willing to pay bank to keep your money safe and as a result the interest rates can become slightly negative (Ilgmann, C., & Menner, M. (2011)).
However, slightly negative does not mean arbitrarily negative. There will be point where the nominal interest rate is so negative nobody will be willing to hold money in deposit accounts. This is trivial just consider an extreme case of negative $-1000\%$ interest rate. Would you hold any money on deposit account under such punitive negative nominal interest rate? I think not! Of course, this raises question what is actually the true ELB if not zero. The studies show that while this effective lower bound will be below zero, it can only go very little below zero - reasonable estimates are around $-1\%$ being maximum (see Hills, T. S., Nakata, T., & Schmidt, S. (2019) and sources cited therein).
Now are there some ways how this issue could be dealt with? Actually yes! Both ZLB and ELB can be easily abolished by abolishing cash. If people do not have choice of withdrawing their money from bank then interest rates can be arbitrarily negative (see Buiter 2009; Buiter 2005; Yates 2004, and Ullersma 2002, for reviews on the relevant literature). Unfortunately, it seems that most societies are not ready to give up cash yet, since despite of support for abolition of cash among experts this idea is not widely embraced by politicians.