Copy text from quick look in El Capitan or Sierra?
It's not that the defaults write command fails. I verified the file,
  and it wrote the variable to it. The problem is somewhere in the
  design. Maybe they have moved it? I have been looking in the
  containers and other files but no luck yet. I hope they didn't
  completely kill it. Maybe there is a way to copy the old Yosemite
  version of QL back? If I figure it out, I will update here.
  https://discussions.apple.com/thread/7250702

TRUE - YES or 1 have the same effect, so, there is no problem on this side.
The option is no longer available on Onyx or TinkerTools too :




Conclusion :

Actually there is no solution to get back the text edit feature on Quick Look.

Following this Apple Thread is the last way to keep hope

It is worth noting that you can copy from spotlight for certain files, as discovered here. Not a great workaround but still allows copying without opening a file. The supported file types are the following:


.doc/.docx/.pps/.ppt/.pptx/.xls/.xlsx (also scrollable)
.rtf including text-based .rtfd
.numbers '09 v2.3, keynote '09 v5.3
.odt
all text based documents, including source code, xml, plists, and .command files
compatible image types