To answer the question, it is elementary my dear Watson. Of course it is, but today I shall cover the accessibility points, but there are far more points at play here.
Software applications are used everywhere. As such, for such a wide demographic, a great deal of software is remarkably terrible. Irredeemably so. A lot of people suffer from colourblindness, poor sight, deafness and dyslexia, and these are all relatively common problems that have. Also a pointer here, even though I'm deaf this isn't about pity this is about establishing a basic accessibility test to tell what is and isn't shit software. Complexities exist, some software is harder to make accessible than others, like Games, but some are incredibly easy, like terminal applications or websites to the point that there is simply *no valid excuse except for the desire to be shuffled into the land of dead software*.
A great deal of the population are colourblind, and this comes in a number of forms. At it's worst it is shades of black and white, so prepare for the worst. Any widespread software that is inaccessible under 2 colours, is automatically trash.
If it requires more than 2 colours, it mandates iconography, and its lack is poor design. Iconography should be used anyway, as it's useful for those of poor sight.
This effects those with poor sight. Poor contrast means you have to strain your eyes to even read the text. Far too many websites have this issue, don't add yours to trash. There are 2 solutions for this, a high contrast mode(fine tuned or toggled, who cares), or just make it high contrast to begin with.
This one is simple. Dyslexic people have a hard time reading. What to do? Allow dyslexia-friendly fonts. No font choices means it's automatically trash. Also some character are identical under some fonts, for example l and I. No font choice? In the bin you go.
This one is also simple, effecting those with poor sight. Narrow fonts, or fonts with incredibly small point size with no zoom-in functionality get thrown into the toilet where it belongs along with the software it labels.
This one is rather specific, mainly for those with poor sight. The nature of the modern web makes screen readers impossibly useless for these poor buggers, as HTML has been perverted long ago to construct bloated and bad structures to support styles and not to support contents. Even nethack, a game from the 1980s has a text to voice reader. If your software can't handle it, or export to it, it goes where it belongs. In the skip.
This ones particularly pisses me the fuck off being deaf. When audio only plays in the left or the right ear, it makes it completely useless to listen to. This usually doesn't effect software, but it sometimes can. There are cases where not having it equal is important, but most don't stand to scrutiny. Unequal or playing in one ear? It gets shoved up your rear.
This comes in 2 flavours. Lack of subtitles, and poor subtitling. For the lack of subtitles, all I can say is it shouldn't be lacking, as it's simple and easy work. As for poor subtitling see what I said about bad fonts and font sizes and colour contrasts. A lot of work fails in this regard.
A lot of these issues are fixed easily by adding configuration options. Some are fixed by using the software for its intended purpose(HTTPS is a Hypertext Transfer Protocol, not a PDF creator, so stop fucking using it like photoshop you toddlers).
If your software meets 2 or more of these "Throw it in the bin" criterion, it is fucking trash, and you should fix it, or fix yourself a gun to paint your wall crimson.
If it meets less than or equal, good stuff. Fix the issues, and your software might suck a little bit less. It will never not suck though, that's the nature of all software.
Also a final note, I have personally broken quite a number of points on this list. A great deal of my own software violates these tests, so the natural conclusion is my software shit. My response? Of course it is, all software is shit, some is just less shit than others.