I manually installed a previous version of the gitg application as suggested here, since I really dislike the new interface.
I then marked the package as "on hold".
After installing updates from aptitude, I found that the newest version was installed again. This doesn't happen when doing an apt[-get] [dist-]upgrade. How can I get aptitude to likewise honour the "held" status of the package?
It turns out that there was an issue with my workflow for installing upgrades from aptitude.
What I always did was to select the "Upgradable Packages" entry and press + to install. This automatically removes the "held" status for any packages, including those for which you manually installed an older version.
Instead, aptitude provides the key U to mark all upgradable packages to be installed, except those marked as "on hold", as expected. (Also available from the menu under "Actions" as "Mark Upgradable".)