Released May 26th, 2020.
Seems safe to install on our Hacks. Ended up with a macOS could not be installed on your computer error message on the Latitude E6230 and 7490 laptops due to bless tool being unable to set the boot disk. But these are like false alarms because the Catalina partitions of those Hacks remained bootable and were properly updated to 10.15.5. The macOS Install Data folders in /System/Volumes/Data, however, had to be manually removed to get rid of the temp partition so, clearly indicative of a minor problem in fully completing the update at the end of the process.
As for 10.15.4, refresh Lilu + WEG kexts and Clover to latest available versions (v1.4.4, v1.3.9 and r5118 at time of writing) before initiating the macOS update. If you cache your kexts from /L/E (or /S/L/E), remember to also update the kexts in the E/C/k/O folder of your EFI partition as the temporary update partition boots only with kexts injection.
Security Update 2020-003 for High Sierra 10.13.6 and Mojave 10.14.6 released too (build 17G13033 and 18G5053).
Edit: 1st June 2020:
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10.15.5 Supplemental Update released today. Build 19F101.