Why discontinue the X.5 LTS strategy?
From Joomla! Documentation
The X.5 LTS strategy prevents the PLT from issuing additional releases on a major release series. If 3.5 were to be the LTS for the 3.x series as previously planned, then any feature proposals or enhancements to the 3.x series would not be able to be merged until development started on 4.0. Now, it is possible to continue development on the 3.x series without locking into a specific release as the LTS release, which enables a longer support period for the 3.x series overall.