Not to be pedantic: since 5.1.0, LWRP is no longer behind preview flag. (regardless, it’s also only available for 2019.1; 2018.3 currently use 4.x-preview branches, due to core engine difference AFAIK.)
I have made argument that LWRP should be supported in 2018 LTS even if with reduced feature, so I won’t repeat it here:
https://discussions.unity.com/t/664083 page-21#post-3873364
https://discussions.unity.com/t/716433 page-3#post-3870511