I tried to PR this twice both in 5.5-dev and release and both were closed and marked as intended removal. According to the feedback they don’t want to support dx (dx11 or dx12) sm5 lumen but continue to support vulkan and metal. It feels a bit odd to me as it’s a very simple thing to add the const back and get parity across all sm5 shader models and rhi’s however they want lumen bundled in with the higher cost shader model (sm6) for d3d only … it’s not even about dx11 really at this point it’s the complete lack of d3d sm5 swrt support at all. This also forces people into the dx12 pso caching pipeline instead of being able to rely on dx11 driver compilation which again dx12 pso has been consistently problematic. 
we have had a very long time to overcome dx11’s shortcomings and know how to handle it. DX12s pso pre 5.3 the and even post 5.3 auto precaching is a headache in and of itself. I’m simply confused at this point because I don’t see Fortnite dropping sm5 or dx11 support anytime soon…. But maybe because Fortnite disabled lumen in sm5 a while back there’s a conflict of interest with PRs that contradict this at the engine level when dumping the Fortnite merge CLs back to the engine release branches. 
Also as you stated the docs and release notes made zero mention and continue to mislead the community as it states DX11 SM5 for SWRT lumen is the min requirement
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