Let me begin with a short introduction. We, professional mathematicians, know that there are many types of journals and sometimes feel that some of them are a bit of spam (it is even common to receive actual spam from some of these journals). I want to concentrate here on well intended journals, i.e., journals that, although being cursed with the unfortunate fact of having low indices, still pursue a noble and honest labour of communicating real mathematics. So here I discard all these predatory journals that end up sending spam to your mail in order to get some payment in exchange of publishing your low quality papers to increase your CV. These are not the topic of this question.
Many of these low impact journals are full of legit math that is considered folklore or already very well-known but unwritten math by professionals. These papers are written by beginners or aficionados that do not know this because of being off the academic conversation. I feel high respect for these people. These are people that may have other jobs and occupations that feed them and their families but still find moments on their busy days to do some math. These people truly love math in a really pure way and, when they are honest, I feel very fortunate and happy for their existence. So I want to make here clear that this question does not pursue criticising them but (on the opposite!) cherish them in any case with good examples. In any case, I do not want to restrict this discussion to math done by aficionados but I want to talk about papers published on low impact journals in general, independently of who are their writers.
If you allow me to tell you a personal anecdote. When I was an graduate student at a top institution I had a very well-regarded professor (having important merits) that would always tell the story of how one day one graduate student of him came with a result that he found not very interesting because he already knew it since years before. He told his student that that was folklore and that it was nothing that deserved a publication. So the student abandoned that path and never published it. Some years later someone else published the result of the student (discovered independently) and that publication ended up being cited in the resolution of a very well-known unsolved problem in the area. This professor always ended his story regretting the fact that he told his student not to publish one result that could had been his best paper ever.
This story is why I think that even small results that are only accepted in these low impact journals are an organic part of math and for this reason these small journals deserve our respect in maybe the exact same way that bacteria or protozoa deserve the respect of the rest of the biosphere. These journals sometimes carry important ideas that could have appear much later if not for them. Sadly, this is something that happens just sporadically and the majority of the papers published on these journals never get a continuation or a citation into the mainstream and aficionados and beginners have to understand that this is the rule. Here I want to talk about the bright exceptions to that rule.
Once I finished with the preamble justifying the convenience of forming this list, let me clarify the actual question:
I am looking for examples of papers published in low impact journals (maybe even unindexed ones) that contained key ideas that were later expanded or used in papers published in higher impact journals (citing of course the initial smaller papers as the seed for that further work).
Let me begin with a recent example of what I am looking for:
The paper On the inequalities for beta function is published in the journal Notes on Number Theory and Discrete Mathematics, supported by the Bulgarian Academy of Science. It is a legit journal publishing real math papers but with very low impact. It is not a fringe predatory journal just a modest, humble one. Some results of this paper were key crucial tools for proving deeper results on the paper, which is published in a highly respected Q1 journal. Thus a paper in a low impact journal climbed up to one paper in a high impact journal, getting much closer to the mainstream of current math publication.
In general, I am looking for these kind of examples. For valid answers you should note clearly the small paper, the low impact journal it was published on, the bigger paper and the higher impact journal where your story reaches a happy end. Just as I did.
