gnome devs (who maintain the livbte library) grew tired of wrestling with making transparency work across all soft/hardware permutations.https://packages.debian.org
search for libvte and read the changelog. The version with a "9" in it drops transparency support. That version is now present in both sid and testing, IIRC.
See whether you can (without breakage) force a libvte package downgrade.The only peeps I know who are working on continuing to support tranparency in libvte are the LinuxDeepin devs.
If you could get a hold of their .deb (or build from their source) I'm guessing you'd regain transparency (if you were lucky to have it previously, under libvte v1.18).
Oops, no. I just revisited (github) and noted that Deepin has only forked python-vte, not libvte.
From reviewing the changelog details, by pinning the older libvte we're not missing anything noteworthy, IMO.
ubuntu has patched libvte to restore transparency.
If you're interested in DIY patching and (re)building libvte, See message #32 here
https://bugs.debian.org/cgi-bin/bugrepo ... bug=729324