Under Prerequisites Windows 7 is not listed.
With the Brave browser and the Google Pixel USB driver can Windows 7 be used?
Is there an incompatibility that prevents its use?

    Manzikert I was able to install GrapheneOS with Windows 8.1 which is also not listed on the supported OS list. Supported in this context just probably means that the Devs didn't test on any others systems but the supported list.

    The good news is that the Pixel driver still supports Windows 7 and that is what's needed to have your phone properly communicate with your Windows computer. I've not tested or used Windows 7, so beware and cannot vouch.

    There's a link to get the driver on the Web Install page. Follows the instructions given on the google driver page to install driver by using Windows Device Manager. If properly installed than phone should be displayed in Device Manager. Get the SDK platform tools at "https://developer.android.com/studio/releases/platform-tools" to check connectivity to the phone by using Android Debug Bridge. Type "adb devices". If your phone shows there; than this is a good sign.

    The next hurdle is WebUSB to work correctly on Windows 7. WebUSB is the technology in Chrome based browsers to communicate with USB connected devices. If there's going to be trouble then this is where it would be when using Win 7. You my have to use the old style way.

    Just saw a note in my Brave browser under Win7 that it will no longer receive updates and that I should upgrade to Win10 if I want further updates. I believe the WebUSB tools in Brave should still work, but am not 100% on that. Barring that, the command line adb tools should certainly work, particularly since the usb driver is available.

    Unless this is mission critical and you can't afford downtime, why not try it out? It's likely the process will stop with an error at the very beginning if it won't work, and you can still use the official Google web tool to reflash the stock OS if needed. No guarantees of course, proceed at your own discretion! :-)

    I couldn't. I have W7 Ultimate SP1 x64.

    I tried several times, with different browsers, changing ports, etc.

    It'd write a partition or two, reboot the phone and it would no longer be "visible" to the computer, so break the process.

    In the end I decided to get my hands dirty and do the process manually, typing the commands in CMD, the old fashion way.

    The hard side is you have to download the image, edit the flash-all.bat file and clean it of everything other than the necessary commands. Then set the phone in fastboot mode and copy-paste the commands to CMD in same order.

    It's an option.