I think this should only be the case for FSDG distributions, of which there is a fairly short list to be seen here:
https://www.gnu.org/distros/free-distros.html
None of these distributions have WebEngine available at the moment, and thus, none of them can have the most recent Sigil release installed. Once the calibre port is complete, they won't have that either -- this is one of the reasons I would like to finish the Python 3 port sooner rather than later, since regardless of my practical disagreement with them, I think it would be great if they could at least freeze on an old version that supports Python 3, rather than freeze on an old version that is stuck with Python 2.
I've provided guidance to the Parabola developers that they should freeze Sigil at the last version to use Webkit, on the rationale that it shouldn't stop being as useful as it used to be... that is all I can do for them, they will either have to freeze packages at old versions or complete the stalled license review of chromium/webengine.