I assume they're referring to streaming-only releases that never make it to home media, aren't popular enough to be seeded indefinitely, or that get removed quickly for a tax break -- e.g.: Disney+ removed Crater ~7 weeks after it was released, purely to reduce residuals and claim losses for lower tax bills[1].
1.A.: https://kotaku.com/disney-streaming-crater-tax-rort-scam-wri...
1.B.: https://www.independent.co.uk/arts-entertainment/films/news/...