If you’re hardcore enough to compile synfig, you can change this line in synfig-studio/src/gui/canvasview.cpp:
synfigapp::Main::set_interpolation(INTERPOLATION_CLAMPED); // Clamped by default.
to whatever value you like and rebuild synfig studio.
In any case, file an issue on github.com/synfig/synfig/issues so that somebody who’s looking for something to fix can implement that option and you can track its progress.
// Sorry that i write in english on french sub-forum, but i decided that posting machine translation doesn’t make much sense
Doesn’t matter what platform you are on. As i said, it’s in synfig-studio/src/gui/ in source code, wherever you placed it to. It currently is not a configuration option, but defined at build time, so you have to rebuild Synfig Studio after changing that.