Sliders are experiencing a resurgence as the form factor has found a new use. In the early days it was used to make a phone smaller and protect the keypad from accidental presses (by putting the keypad on a slider), today they are used to maximize the screen-to-body ratio - there's no more keyboards, it's the selfie camera that's on the slider. Sliders hit their peak in the 2007-2009 period. A whopping 109 different slider phones were launched in 2009. However, as the chart below shows they experienced a steep decline soon after. The popularity of the form factor peaked in the same...
Source: https://www.gsmarena.com/counterclockwise_the_rise_death_and_rebirth_of_the_slider_phone-news-35034.php