I would think by the time older LTS distros are up to C++17, that Scintillua/LongTerm3 will have no reason to exist anymore.
Old LTS will never get it, thats why they are called "stable", what I meant is the next LTS, for example Ubuntu is scheduled to release a new LTS next month which will have gcc7 in its repositories even if it uses gcc 6 by default.
IIUC it's only
the new maintainer of that branch needing to stick to C++11 for his own project(s) for now and so can't use the tip of Scintilla code. I assume that eventually the goal is to have Scintillua into the mainline or some future external library containing all of Scintilla's lexers (see scintilla-interest mailing list).
Or at least support it there.
Cheers Lex