Disclaimer: Older versions lack security updates for vulnerabilities discovered after 2018. Do not log into sensitive Google accounts on KitKat devices if you use them for banking. Use a dedicated "media only" account for YouTube.

require Android 5.0+, older legacy forks or versions are widely used to bypass Google Service requirements while offering background playback.

You'll see a "Update YouTube" nag once in a while. Ignore it. And some new video formats may not play (rare on older content).