I also just got my Dell U4919 DW and having the same issues as described here.
First of all, if you are using the cables provided with the display you should absolutely be able to get the full resolution on supported Mac's (e.g. I've a 2016 MBP 13"). I validated this by installing Windows 10 in BootCamp and I was able to select the full 5120*1440. Looks like Windows somehow get the full resolution from the display channel or does have a plug & play driver in place - idk. macOS Catalina beta 1 also doesn't seem to fix the issue.
I did some analysis on that issue and probably found the root cause:
Displays communicate with the client by sending an EDID (Extended Display Identification Data). This 128 byte long data contains all informations about the display, like manufacturer, serial number, supported "standard" resolution and the native resolution.
The EDID of the U4919DW (and likely all other displays with resolutions greater than 4096 pixels ) contains 3840*1080 as the native resolution. Thats not a fault by the manufacturer but a limit of the EDID standard. The data field which contains the native resolution is 8 bytes in length for native high and width of the display. Therefore there are 4 bytes for high and 4 bytes for width which leaves us with a max value of 4096 (8^4).
Since apple already supports a variety of displays with resolutions higher than 4096, there must already be a technical solution for these displays, right?
It turns out, there is - its called "display overrides".
Display overrides are placed in /System/Library/Displays/Contents/Resources/Overrides (SIP Protected Folder)
With way over 300 display override files, Apple actively using this technology to modify EDID's to get "problematic" displays to work correctly.
Just overriding the EDID (which would be easy since those files just contains the VendorID, ProductID and the "custom" EDID) won't work in this case because the EDID data field is to small for the value.
Apple seems to have an additional technology for this case called "overlay" which are defined in .mtdd files which are also located at the path mentioned above. It looks like they even use .mtdd files for the LG 5k Ultrafine displays.
Unfortunately I was not able to find any documentation for the mtdd overlay value (seems to be 384 bytes long).
Therefore we now know the root cause and the theoretical solution.
Maybe there is someone out there which knows how the overlay value in the mtdd files is calculated?
I also did open a Radar bug report providing vendor, displayid and the EDID of my U4919DW.
As a fix might be implemented very easy, hopefully Apple will implement this soon.