This issue is in LightRoom Classic version 11.5. My computer is a Windows 10 operating system.
I would like to have the export dialogue for sizing images changed back to the way it was. It is a project to convert from Megabytes to Megapixels accurately. Recently I had to find how many MP would equal 200MB, which took several tries. And then, it turned out it was a little bit more than 200MB and the website would not accept my images until I lowered the amount. Not sure why anyone thought it was a good idea to use MP's instead of MB's. In my opinion it was a BAD idea.
I may be misunderstanding your post -- the Limit File Size To option is still available in the File Settings section of Export options, as shown in your second screenshot. There haven't been any changes in that respect. Does it not meet your needs?
I think it's worth noting that you have .tiff selected in the first screenshot and .jpeg selected in the second. You also reference Image Resizing panel in one screenshot, and the File Settings panel in the other.
I think if you select jpeg as an output file type instead of .tiff, you'll see the option you are looking for 🙂