Periodically I’m tempted by the ultra wide monitors on the market. If I buy one, it’ll be the 29” LG but my question is can the new pi 3b+ do 60 Hz at 2560x1080?
All Pi models have the same Video Core, so the Pi3B+ is no different to earlier models in this respect.
There are some limitations in the modes which can be supported due to the pixel clock limit. See https://elinux.org/RPiconfig#Video
I suspect you are out of luck; even though the rate may be supported there seems to be no wide screen mode - it is possible to create custom modes. I have never seen a screen with a 64:27 ratio!
You can try this. with support for 2560x1080 resolution on LG25UM65 display. 2560x1080