The constant talk about sexism, racism, ageism, etc in our industry almost completely disproves "STEM shortage" argument.
If such people exist, it won't be long before someone realizes they can hire these people cheaply because they are less in demand. Undercutting your labor costs gives a massive competitive advantage bringing products to market. There are thousands of startups every year started by a very wide variety of people from widely different backgrounds.
Ageism is a particularly good canary in the coalmine. If someone has decades of relevant experience and you can't find anyone, why wouldn't you hire them and if they've worked for your company for years and know all the subsystems well, why would you ever think of firing them and hiring someone else?
This frivolous misuse of human capital shows that overall, there must certainly be a surplus of talent rather than a deficit.
A great counterpoint to prove this is/was the web boom. Web talent used to be priced at $50-60K because such people were everywhere. Once Javascript became fast and highly-interactive websites and web apps became hugely in demand, a localized deficit started.
The reason for the localized deficit is obvious, but worth reiterating. The development tools and language were considered frustrating to learn and use by other programmers. There was a "not a real dev" stigma. IE and other cross-platform issues were hard to deal with. Powered by the new, fast JS engines, a glut of new APIs surfaced which led to another glut of CSS features -- all taking almost super-human effort to keep up with. Complicating this, most frontend developers up to that point didn't have strong programming skills, so you had either experienced programmers or knowledgeable frontend developers with a very small overlap.
The result has been to hire everyone. In 2010-2015 especially, if you had 50 people apply, probably only one person qualified (and if you found two, it was worth considering hiring them both). With real choices being that limited, companies simply cannot afford to waste time to market trying to find a candidate that "is the right fit" or whatever the BS "ism" label is in that area. Even people with no programming experience outside of a couple months of "boot camp" have a chance at getting hired.
As time has gone by, that drought has become at least mostly slaked. Companies can be a little pickier about whom they hire and wages have mostly stabilized at a bit over average programmer rates (which seems to account for the extra troubles of frontend development over most other types of programming).
I'm betting that once the current tech bubble bursts and wages plummet, we'll still keep hearing about the shortage of programmers who will work for $40K per year.