But practically speaking, in the real world we reside in, approximately no one authors peer reviewed research without at least being in graduate school. You frequently see "talks" at conferences given by non-PhDs, but it's exceptionally rare to find math or computer science research published in reputable journals or conferences by non-PhDs.
The dirty secret is that the formal barrier to entry was never really the problem. It's extraordinarily difficult to learn how to conduct meaningful research. The structure of a PhD is imperfect, but it places you in direct collaboration with an experienced researcher who can mentor you. Half of the entire work of a PhD program is precisely learning how to learn on your own so you can find interesting research questions.
I don't think it's productive to talk about what's merely possible. A more realistic barometer is what's plausible. Anyone who can bootstrap themselves up to original research in math or computer science through independent study is extremely gifted. You don't want to set a goal that requires you to be extremely gifted right from the start.
Additionally, nothing has been done to separate the issue of correlation and causation. If research capable individuals pursue a PhD because it interests them, who's to say that their research capability came from the PhD versus it being intrinsic.
One can learn a lot from a PhD. But saying that it's a practical requirement for research in this modern era of information access is without definitive evidence.
The "all information is free and you can study anything using just a computer" is a stupid meme that doesn't work for the vast majority of people.
The parent commenter seems to be suggesting that a PhD isn’t a requirement to work successfully on interesting research problems. It’s possible to go online and read the same papers everyone’s reading. You can still talk to people to get guidance and find out known things that are not published.
Sure you can learn a lot from just the literature but in my experience there is a set of heuristics and common understandings/techniques that are often passed on through instructors or peers you're collaborating with that are absent in textbooks. More importantly, points of emphasis and exigent problems in an era of information deluge often come through only when in regular contact with practitioners and attendance of conferences.
These are thoughts that have crystallized for me over a 4 year period of starting math late in undergrad, struggling through and putting a lot of time into non essential aspects in the process of self study to get up to speed, hitting a road block when attempting to self study graduate level material but ultimately managing upon working in an academic setting.
It's technically possible, but it's unrealistic. The work that is published in journals often demonstrates results that are known by the community years before the actual publication. There's effectively a "secret club" in many disciplines that you need to have connections to in order to actually understand the current state-of-the-art. Without those connections, it's hard to understand what is high-impact and meaningful. This means the research you would do is likely to be insignificant, or already done and pending publication.
An easy way to avoid this is to start attending conferences and working with establishment players, which is exactly what doing a PhD entails.