If your embed code contains embed.figma.com instead of www.figma.com, it means that Figma Embed KIT 2.0 is used. Although the hide-ui parameter is not specified in the documentation, you can still use it. Just use &hide-ui=true or &hide-ui=1 instead of the encoded string %26hide-ui%3D1. Or use the documented query parameters.
Learn more on Figma Developers:
https://www.figma.com/developers/embed#embed-a-figma-prototype