902 security vulnerabilities published for Android 9, 179 of which are “critical” (9+) vulnerability scores.
Including “This could lead to remote escalation of privilege with no additional execution privileges needed. User interaction is not needed for exploitation.”
Yeah, no thanks.
And if you stay that many versions behind, you carry a gold mine of vulnerabilities that can be exploited. Your phone might work, but it’s far from a safe or good idea.