The decline in support for RSS is more visible on the client side. Sure there are reader options for the determined, especially if technically minded, but that's a marginal population.
In today's hyperconcentrated digital landscape about the only thing that matters for mass market relevance are the default options on client software controlled by gatekeepers.
Mozilla too has abandoned RSS, which may or may not be correlated with them being (alas) increasingly irrelevant.
Eventually lack of popular RSS clients lead to slow decline on the server side as well. A new and fancy website today using a "modern" stack more likely than not does not support RSS, its fully aligned with the "follow us on XYZ" mentality.
I found a blog yesterday, from a well-known indie gamedev, with no feed at all. No RSS, no atom. It was quite shocking because the vast, vast majority have one.
I didn't say "decline in support for RSS" but there are many examples of them being removed.
Twitter dropped RSS completely. Facebook dropped them for pages. And then other large sites (Youtube I think is one if I recall right) made it less of a public feature.
Twitter is one that's pretty annoying because there is a lot that gets posted there first before it's published or not published elsewhere at all.
