I’m going to use three fields as an example: computer science, biology and paleontology. How do keep with interesting developments in each of these fields? Which journals are the most reputable in their respective fields? How do I know that the journal is reputable? If the journal is reputable, how do I know that a particular paper isn’t a fraudulent paper that managed to slip through the cracks?
Physorg is a great resource for someone who is not currently fully educated in the field but has a strong interest. They do really good summaries of each topic and provide just enough context to go and find out more. They also do have good RSS feeds available, so you can easily use whatever client you like to get their content.
As for FOSS clients, just have a look in F-Droid and you will find a bunch. I use Feedflow at the moment but I have tried a few from F-Droid and they are all similar. On desktop I would recommend looking at a different one depending on your desktop environment. On EndeavourOS with KDE I have used a few but be one included with the Kontact suite is fairly good.