The Alert Slider is a hardware-based, 3-step switch that allows the user to mute a given smartphone quickly and easily. It is exclusive to the brand OnePlus - but not for long, according to an increasingly prominent leaker. They now assert that Realme will also start using it in its devices, starting with an upcoming mid-ranger in 2022.
https://www.notebookcheck.net/OnePlus-Alert-Slider-will-show-up-in-a-new-Realme-mid-range-phone.606794.0.html
Actually the original OnePlus One never had an alert slider. It was the OnePlus 2 that introduced this wonderful concept.
Could not be any happier to see it be adopted by more brands. Should be a staple on all phones!