I don't believe this would be a suitable limitation for this site. I am an academic researcher in programming languages, so I'm going to meet this bar, but many people who are nonetheless serious PL developers will not, and many people who may not be "serious" developers nonetheless have serious contributions to make.
Industrial programming-language development is in any case also not so clear-cut as quantitative finance seems to be: it's quite common for developers whose primary focus is elsewhere to be building domain-specific languages — both internal and external — and quite reasonably to have design questions about these, yet it's not clear that they "work on languages in serious capacity". I would not want to exclude these people.
For a non-industry example, let's take the Emily language: it's a hobby project from I believe a professional game developer, but it's an exploration of language model that I take very seriously. Another I have referenced previously is Om, another spare-time exploration of a non-commercial space. There are numerous others in a similar capacity that I have cited in my academic work. I wouldn't want to exclude those either.
There are even academic programming-language communities that are not interested in "VM performance, advancements in type systems, compiler optimizations, provable transformations": all sorts of work on visual or block-based languages doesn't overlap with those, for example. They ask and answer, and publish, ergonomic questions that are in many ways parallel to syntax in a textual language. On the other hand, anyone who is interested in those topics should be welcomed here, and ideally not put off.
A lot of my work is in the area of novel programming interfaces, and a large amount of novel development and exploration in that area also comes from non-professionals, including those presenting at academic venues. I don't think that this system is more meaningful than this one because it's from me and the other (appearing in the same venue!) is from, I think, a non-academic. It isn't about who's doing it, but what they're doing.
Focusing on the people is looking in the wrong place.
On the other hand, there have been a lot of what amount to engagement-bait questions. Many of these fundamentally come down to:
- "make a list of languages you've heard of and show a syntax example"
- "jot down anything you can think of about this and label each 'pro' or 'con'"
- "tell me why this controversial thing is bad"
These do not lead to constructive answers, and should not continue. That is a property of the question, not the people involved, and in many cases it seems to be out of a reflexive avoidance of the superficial appearance of an "opinion-based" question. It's also been reinforced systemically because so many people have an easy answer to give, and they upvote the question when they do so.
So far, these questions have outnumbered and out-engaged others, and I do agree that drowning out other topics is undesirable.
Good answers should provide synthesis of information and analysis of trade-offs made in concrete choices, and good questions should facilitate those answers. Those questions could come from anyone, whether making a hobby project or not, and many fundamental questions arise equally across the spectrum. Different people will be able to give good answers to different questions, and that's fine too.
Setting the scope of questions and answers is good, and still in the process of refinement (I hope), but setting the scope of participants is not the right way to go about that.