Historical releases of XFCE are in branches here:

• xfce-1.10
• xfce-1.11
• xfce-1.11b
• xfce-1.2.4
• xfce-2.0.1
• xfce-2.4.0
• xfce-3.0.2
• xfce-3.2.1
• xfce-3.2.5
• xfce-3.3.2
• xfce-3.4.0
• xfce-3.4.4
• xfce-3.6.3
• xfce-3.8.14
• xfce-3.8.16
• xfce-3.8.18
• xfce-common-2.4.0

The bxform dependency for versions dependent on XForms can be found at:
ftp://cpan.sai.msu.ru/pub/unix/libs/X11/xforms/linux/

The original-tarballs branch contains the original tarballs used to collect the source for the remainder of the branches.

Sources for tarballs include:

• 1.10, 1.11, 1.11b, 1.2.4, 2.4.0: http://www.mediafire.com/file/9uecrqtyxprlppf/XFCE.7z/file
• 2.0.1: http://distrib-coffee.ipsl.jussieu.fr/pub/linux/mandriva-prehistory/6.0/SRPMS/xfce-2.0.1-6rh5.src.rpm
• 3.0.2: https://archive.org/download/Mandrake_6.1/disk02.iso/xfce-3.0.2-7mdk.src.rpm
• 3.2.1: http://archive.download.redhat.com/pub/contrib/libc6/SRPMS/xfce-3.2.1-1.src.rpm
• 3.2.5, 3.4.0, 3.4.4, 3.6.3, 3.8.14, 3.8.16, 3.8.18: http://ftp.twaren.net/BSD/OpenBSD/distfiles/?C=N;O=D
• 3.3.2: http://distrib-coffee.ipsl.jussieu.fr/pub/linux/mandriva-prehistory/7.1/SRPMS/xfce-3.3.2-2mdk.src.rpm

Thank you to u/grem75 for digging up many of these, including the very difficult to find 1.x and 2.x versions.