Has App Store download size limit over Cellular data changed from 150 MB in iOS 12?
No. The upper cap for downloading new apps over mobile data connection is unchanged from Apple. It's still at 150 MB. There is no official word from Apple after last revising the upper cap to 150 MB. This is the warning that I get (Indian App Store, accessing over 3G data connection in India) when attempting to download an app of size 2 GB.



However, the related Apple support document, If you can’t download or update apps, or if apps get stuck mentions the following in fine print.


  Connect to Wi-Fi
  
  If you’re using cellular data or Wi-Fi and have issues with an app, try to connect to another Wi-Fi network. You can use cellular data for apps up to 150 MB* in size.
  
  
  
  Get more help
  
  If you still have issues with apps on your device, contact Apple Support.
  
  *Depending on your country or region, and your carrier, the download limit might be higher.


The restriction is in place not due to some limitations in iOS, but to guard the mobile service providers network against congestions. From the statement it appears that some service providers must have worked with Apple and are offering a higher download size cap.