According to a report by Bloomberg published earlier today, YouTube will start limiting its streaming video quality by default, bumping everyone in the world down to "standard definition" quality — or 480p. However, higher resolutions/definitions will still be available for manual selection.
YouTube previously made this switch for viewers in the European Union late last week, and other streaming services like Netflix, Disney Plus, and Apple TV+ have made similar moves to reduce strain on European network infrastructure. With much of the world in social distancing/quarantine mode, we're watching a whole lot more streaming services.
The change is set to happen "over the coming days." I should note that at the time of writing, YouTube is still defaulting to an HD-quality 720p stream for me.
In a statement provided to Bloomberg, Google said: "We continue to work closely with governments and network operators around the globe to do our part to minimize stress on the system during this unprecedented situation."