I think there may be a misunderstanding. libevent-2.0-5 is not a package version, it's the name of the package, while the version number is 2.0.21. You can check it as follows:
# apt-cache showpkg libevent-2.0 Package: libevent-2.0-5 Versions: 2.0.21-stable-1ubuntu1 (/var/lib/apt/lists/us.archive.ubuntu.com_ubuntu_dists_trusty_main_binary-amd64_Packages) (/var/lib/dpkg/status) ..... We can double-check this: the command apt-get install packagename=packageversion installs the specific package version.
# apt-get install libevent-2.0-5 Reading package lists... Done Building dependency tree Reading state information... Done libevent-2.0-5 is already the newest version. upgraded, 0 newly installed, 0 to remove and 0 not upgraded. # apt-get install libevent-2.0-5=2.0.21-stable-1ubuntu1 Reading package lists... Done Building dependency tree Reading state information... Done libevent-2.0-5 is already the newest version. 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded. Your only worry need be that future system updates do not involve libevent-2.0-5. You can stick to this version of a package with the following command:
apt-mark hold libevent-2.0-5 and then, when you do want to to update the package,
apt-mark unhold libevent-2.0-5