Have there been studies of differences between journal paper authors and conference paper authors? In research fields such as computer science, academic publication mostly happens at two kinds of venues: peer-reviewed journals, and peer-reviewed conferences. The main difference is that conferences usually require that some author presents the work at the conference and pays a registration fee for the conference, whereas journals do not.

One criticism of conference in this light is that they are not open to all academics: they exclude researchers in poorer countries and institutions which cannot afford the conference registration fee or trip, they discourage geographically remote authors for which attending a conference is complicated, they discourage people who for personal reasons cannot easily travel (caring for a child or family member, having a disability, etc.).

My question is: has there been any systematic study to estimate the magnitude of this effect? E.g., take the dataset of article metadata for articles and journals of various fields, and study if there are systematic differences in terms of the author's institutions, countries, gender, etc.
My relatively quick Google and Google Advanced searches didn’t identify empirical studies that address the question you ask. However, a search using the terms [scientific conference presentation] and [diversity] yielded this article discussing the need for actively working to increase diversity among conference presenters:
https://physicsworld.com/a/fifteen-tips-to-make-scientific-conferences-more-welcoming-for-everyone/
This article does not address your question directly, but it makes me think that framing your question in terms of a comparison between authors of published papers and authors of conference papers may not be the most productive approach to finding relevant literature.
The majority of researchers who have published in peer-reviewed journals have also presented their work (sometimes the same work) at conferences. By making the author/presenter the unit of analysis, one risks substantial overlap in the two groups—-very likely washing out any real differences that exist. “Noisy” data are problematic in their potential to overlook effects as well as to suggest them erroneously.
I think that a reasonable answer to your question would be more likely to come from separate examination of the two avenues (journals vs presentations)for the diversity of their authors.
In 20+ years’ experience with both conference presentations and peer-reviewed publications, I note barriers to diversity in both venues. It is certainly true that attending a conference takes time and money that many researchers lack. However, research of sufficiently high quality to obtain inclusion in peer-reviewed journals typically comes from well-funded labs to which only a small percentage of the academic world has access. Biases in admitting students into graduate programs, biases in hiring academic/research personnel, and biases in the awarding of research grants guarantee that both journal and conference avenues will have less than optimal diversity. I think that separate investigations of the two would be very interesting.