If you previously bought/downloaded an app for (not WP8.1 or earlier):
Several community-driven archives provide .xap files for sideloading. Examples include: Download Windows Phone Apps -XAP APPX Files- to PC
He closed his laptop, the Archive’s index shimmering in the screen’s reflection. Outside, the city moved on—new phones, new icons, new stores—but in his quiet corner, the past kept running. The apps behaved like notes in a bottle: tossed into time, then rescued and read, again and again. If you previously bought/downloaded an app for (not WP8
However, "dead" does not mean "useless." There is a thriving preservation community dedicated to archiving (for WP7/8) and APPX files (for WP8.1/Windows 10 Mobile). The trick is learning how to download these application packages directly to your Windows PC before sideloading them onto your phone. the city moved on—new phones