Thanks for your reply, @Yochanan manjaro-alsa is installed on my system, but it seems that it depends on alsa-oss, without requiring lib32-alsa-oss. Anyway, I was able to remove lib32-alsa-oss and lib32-lz4.
Regarding the 6 xfce packages, my system does not have the -gtk3 ones. Is it necessary to install them ? Currently, $ sudo pacman -S xfce4 returns:
:: There are 15 members in group xfce4:
:: Repository extra
1) exo 2) garcon 3) thunar 4) thunar-volman 5) tumbler
6) xfce4-appfinder 7) xfce4-panel 8) xfce4-power-manager 9) xfce4-session
10) xfce4-settings 11) xfce4-terminal 12) xfconf 13) xfdesktop 14) xfwm4
Enter a selection (default=all):
Thanks for your help, @Yochanan, I marked all those packages at explicitly installed.
Searching for meta-transfer-pkg in my repositories still lists the suite of -gtk3 packages, although they are not installed.