The short answer is:
Windows 8.1 for Teams is like using a VCR to play a 4K Blu-Ray. The plug fits physically, but the magic isn't there. It’s time to let go of the tiles and upgrade. Suggested Image for the Post: A split screen. Left side: The Windows 8.1 Start screen with colorful tiles. Right side: A Microsoft Teams error message saying "Unsupported OS." microsoft teams for windows 8.1
If you need Teams on that PC, your only safe, practical option is to (using the old free upgrade trick still works) or install Linux and use the Teams PWA. The short answer is: Windows 8
Windows 8.1 was the bridge between the disastrous original Windows 8 and the polished Windows 10. While only a small fraction of users remain on it (mostly holding onto legacy hardware or specific touch-screen workflows), a question pops up in IT forums surprisingly often: Can I run Microsoft Teams here? Suggested Image for the Post: A split screen