Saw this recently and immediately thought this is something GT Sport did wrong by having larger updates, especially towards the beginning.
It's taken from a recent GDC presentation by Bungie:
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Essentially by Polyphony giving us large amounts of content early on, players then set an expectation for content that simply wasn't sustainable long-term. The content stockpile was eventually used up until we get to what the actual production rate is.
The drop-off is why it seems worse than it actually is. We're still doing better for additional content than e.g. GT6, and GT5's new content we had to pay for on top of that.