[Update October 2020] Oracle will introduce an extended support offering for Oracle Solaris 11.3 for a limited time. Details can be found in the updated Lifetime Support Policy document for operating systems and hardware and this blog article
After July 2020 you will not get any more LSUs (Limited Support Updates) for Solaris 11.3, so it is time to move to Solaris 11.4. If this is old news to you you can safely stop reading this article, we have stated that publicly e. g. in MOS doc 2382427.1
The LSU releases were never an equivalent to the SRU (Support Repository Update) releases we continue to provide, they always were ports of just a subset of fixes to Solaris 11.4. The only intent to offer the LSU train was to give our customers more time to upgrade to 11.4 since with Solaris 11.4 we switch to a continuous release model. This "continuous release model" contiuously prorvides feature enhancement in parallel with bug fixes to Solaris 11.4, for details see Oracle Solaris Moving to a Continuous Delivery Model (January 2017) or "Continuous Delivery, Really?" (December 2017)
Why do do we stop that LSU train now? One main driver is python. Solaris, like many other software products, makes extensive use of the Python programming language, it used to use python 2.7 a lot, but python 2.7 is coming an end, cf. the "official" announcement "PEP 373 -- Python 2.7 Release Schedule" , or the nicely animated countdown clock. After January 1st 2020 python 2.7 is considered "unsupported", and without upstream support for python 2.7 we cannot rely on python 2.7, and thus had to move Solaris 11 to python 3. This migration has been happening in Solaris 11.4 and will continue in 11.4. Oracle has also published the end date for the LSU train for Solaris 11.3 in its support docs, cf. the MOS doc "Oracle Solaris 11.3 Limited Support Updates (LSU) Index"
How can you check if you can seamlessly upgrade your Solaris 11.3 environment to Solaris 11.4? With the release of Solaris 11.4 we also delivered a tool that allows you to check if your environment could have issues when running under Solaris 11.4. This tool is commonly called "Enterprise Health Check", and there is the MOS doc Pre-Update Enterprise Health Check Tool execution example
So do not be afraid of upgrading to Solaris 11.4 even if you tried before and faced challenges. We are well aware that we to aggressively removed functionality from Solaris 11.4 when we released it in the first place (wrongly assuming that it is not used anymore) We have brought back a number features, e. g. libucb which missed in the first release of Solaris 11.4 and with the current SRU13 there should be more issues of that kind, and we consider any occurrence of such an issue worth a SR