Hi, JC_28!
Thanks for reaching out to Microsoft Community and posting your inquiry regarding the Surface Pro 7's compatibility with ultrawide monitors with a 21:9 aspect ratio. Thanks for taking the time to let us know about this and allowing us to help you out.
Based on the Troubleshoot connecting Surface to second screen page, the maximum resolution rate of the Surface Pro 7 is 5120 x 3200. This is confirmed when I checked the specs of all 3 Intel Processor Graphics for the Surface Pro 7 inside the device. Its max resolution is what is stated in the table on the link provided above. If you scroll down on that page, you will see compatible monitors there. You may be able to connect your device to an ultrawide monitors, the image/video you see on the monitor may be adjusted to what the device is capable of.
We hope this helps you out.
Kind regards,
Janette_Z