In my experience that is quite rare though. Probably less than 10% of the downgrades you'll do will actually not work.
On the other hand if the app has important data you want to preserve, the other ways of doing so are a) hunt down the apk manually, with the fdroid website not having a convenient download button for older versions, or b) use something like neo backup to make a backup, uninstall, install the older version, and revert the backup except the apk. Both are 1 minute for what could be one button press.

Edit: looking at the fdroid page the download buttons are there now, still you need to search up that page, it'll probably still take a minute using that method. Why use fdroid if you need to google apks like a caveman in the end anyway?