> It uses modern C++17+ code

Ha ha ha, that's funny. It uses pre-98 C++ code, that's set in stone because of extension/UNO APIs. Yes, you can use C++17 in a bunch of places, but not for the basic structures, classes, idioms etc.

And - that's coming from a huge LibreOffice supporter. Speak at conventions, got the T-shirts, everything.

You are referring to the UNO API. The internal code is most definitely not stuck in "pre-98 C++ code".