Microsoft Office is frequently updated to patch security vulnerabilities (CVEs). A portable version is a "snapshot" in time; it cannot be updated via Windows Update. If a critical security flaw is discovered in Word 2019 next month, the portable user remains vulnerable indefinitely, creating a security hole on every machine the USB drive touches.
To understand why, you need to know how Office 2019 differs from older versions: microsoft-office-2019-portable
Free, official, and browser-based access to Office tools. Microsoft Office is frequently updated to patch security
Unlike Office 365, which requires internet access for activation and many features, Office 2019 works completely offline. To understand why, you need to know how
In an era where remote work and cross-device productivity reign supreme, the idea of carrying a fully functional office suite in your pocket is incredibly tempting. The search for a version has exploded in recent years. Users dream of plugging a USB flash drive into any computer—a library PC, a hotel business center, or a friend’s laptop—and launching Word, Excel, or PowerPoint without installation, administrator rights, or leaving traces behind.
Continuing to use any version of Office 2019—especially unofficial portable versions—after this date increases your vulnerability to viruses and malware. Microsoft Learn