I doubt if it is even possible for anyone to make rigorous claims about broad software quality over long periods of time. The common discourse over this subject merely consists of sharing anecdotes and speculative pet theories. To answer this question convincingly one would ideally need to take a comprehensive and representative sample across the whole range of software from the past and today and make comparisons across the vast categories of software that exist. Doing this even for today’s software would be a tremendous undertaking for a variety of reasons. Now consider doing it for the past where one must face the simple fact that the vast majority of historic software is lost or incomplete. Even if we have binaries we might not have the source code. Next we may not even have a computer to run the software on. The software that does survive might not be a representative sample of past software-- it may have experienced survivorship bias due to its quality or for some other reason.

The vast majority of software (past or present) is not distributed and therefore difficult to access. So I find when people talk about “software” in this context they often mean commercial “boxed product” software or software that otherwise has an end-user that is the general public rather than “all software”. That is simply the only neighbourhood of the software universe that most people are familiar with. Yet even if we were to restrict ourselves to that category of software there is a complete lack of convincing comprehensive comparisons. And even if there were it might only be an indication of a trend confined to that category of software.

Most commentators suggest the trend concerning software quality continues and may even accelerate in the future. So instead of trying to compare the present with an inaccessible past, why not compare the present with an eventual future? Design objective metrics and/or recruit esteemed judges for subjective assessments, then assess today's software and repeat this assessment a number of years later and compare the metrics.