Mac pro 5.1 Usb - c PCI card

Yes, here is one

https://eshop.macsales.com/item/NewerTech/MXPCIE4U3/?utm_source=google&utm_medium=shoppingengine&utm_campaign=googlebase&gclid=Cj0KCQjwj9LkBRDnARIsAGQ-hUfCUVPSMTZiSGDYcj5xSduRaVXH4boCcoigyckMopwB6TE-iICfOMYaAid3EALw_wcB&gclsrc=aw.ds

Note:

• If using a Mac, you may experience disk ejections when waking from Sleep mode. This is a known issue that occurs when using add-on PCIe cards with the Mac operating system. It is suggested that you manually eject USB devices prior to enabling Sleep mode on a Mac. This issue is not present with Windows
• Boot Camp is not supported

USB-C is the name of the connector, ONLY.

These cards do not, by any stretch of the imagination, provide ThunderBolt-3 or ThunderBolt of any description. At most it can provide USB 3.1, even if you were to find one with a USB-C connector.

Here is a USBC card for Mac

https://eshop.macsales.com/item/Sonnet%20Technologies/USB3C2PME/?utm_source=google&utm_medium=shoppingengine&utm_campaign=googlebase&gclid=CjwKCAjw1dzkBRBWEiwAROVDLKLIQF1LNvwHczJkNvs67vHSwYYxqeU2zCPOH1j6_GzQY0CLzEe2bhoCHK0QAvD_BwE&gclsrc=aw.ds

Since your MP does not have Thunderbolt, all you get is 10 Gbps USB transfer speeds with a USBC connector

There are now a handful of Mac Pro compatible USB3.1 (with USB-C) cards. These as others have mentioned do 10Gbps but do not and never will support Thunderbolt.

The Sonnet card at least does support Windows and Linux as well as macOS although it is not made absolutely clear whether this includes via Boot Camp. I see no reason why if it works in a real Windows PC why it would not work in a Mac Pro via Boot Camp.

Note: USB3.1 Gen1 is effectively USB3.0 or vice versa. So the NewerTech card is perhaps being economical with the truth. USB3.1 Gen1 aka USB3.0 is only 5Gbps. The Sonnet and Caldigit cards are USB3.1 Gen2 cards with a USB-C connector and hence support 10Gbps.

Sonnet makes a USB-C card also, as does CalDigit.