Universal Control 1.7.4 Officially Supports the following Operating Systems:
(...) OS X 10.11 (El Capitan)
Current App versions compatible with this Universal Control release:
SL Remote 1.5 for iPad
QMix 1.3 for iPhone and iPod Touch
(Quotes from the release notes of UC 1.7.4)
Maybe it's time to buy second hand iDevices for the use with the legacy boards and keep them in the configuration needed. We need to consider that the legacy software is not developed further, so we have to use the "matching" hardware. The iDevices go the way the go, the legacy StudioLives and their software can not follow any longer.
It may be worth to open a ticket to get an official statement from PreSonus on this subject.