I try to understand the metadata.txt file to write a shell script for maintaining updates.
I’m a little bit confused if I misunderstand the documentation Build Metadata Reference | F-Droid - Free and Open Source Android App Repository or for some application (see below) the build software is since several years not published to the repository.
MPDroid
I have taken a look at Files · master · F-Droid / Data · GitLab
There is:
Current Version:1.07.2 Final
Current Version Code:54
According to Build Metadata Reference | F-Droid - Free and Open Source Android App Repository
1.07.2 Final should be the freshest version.
So I tried to fetch: https://f-droid.org/repo/com.namelessdev.mpdroid_54.apk - but it isn’t available 
I compare this now with: MPDroid | F-Droid - Free and Open Source Android App Repository
Here I see that the Version1.0 with Version Code 20 is advertised (from 2012)
What is the missing point here ?
Book Catalogue
Another look I have taken was: Files · master · F-Droid / Data · GitLab
Here I see:
Current Version:5.1.1
Current Version Code:168
According to the Build entries I understand that this version build is disabled.
So my next Idea was to read reverse the build entries till I found the first not-disabled entry.
This would be here the Version 5.0.7 (Version Code: 151).
As above for MPDroid I can’t either download the Version Code 151 (or 168)
So I check again https://f-droid.org/packages/com.eleybourn.bookcatalogue/
Here I see that the Version 3.8.1 (Version Code 70) from 2011 is the last available version.