Ran into this one while investigating the crashes of a DockBarX plugin. apt gives me the following:
$ apt show xfce4-whiskermenu-plugin
Package: xfce4-whiskermenu-plugin
Version: 2.1.5-0ubuntu1
Priority: optional
Section: universe/xfce
Origin: Ubuntu
Maintainer: Xubuntu Developers <[email protected]>
Original-Maintainer: Debian Xfce Maintainers <[email protected]>
However, developer's site does not even list version 2.1.5. The oldest version mentioned in the release history is 2.3.0 (2018/09/30); 2.1.5 was probably 5+ years ago.
So, is version 2.1.5 in the Ubuntu repository guaranteed to be the 2.1.5 developer had long ago? Or is the Ubuntu repo using its own, unrelated numbering?
Back in 2018, version 2.1.5 (at the time) was manually uploaded. It's possible the developer's website has changed since then.
Why this was version bumped to 2.1.5 independently of a Debian sync is unknown to me, but this 'version' is specifically only in 18.04 Bionic Beaver - later versions of Ubuntu have newer variants as listed on its Launchpad page.
Once an Ubuntu version releases, it is rare to see packages have major version bumps after the fact. Because of this, 18.04 (which was released in 2018) has the old version of the package. You will probably want to upgrade to a newer release (20.04 once it's released in a couple months) to get 'newer' software versions.
In this case, it's Debian developers who package it in Debian and it syncs down to Ubuntu during dev cycles.