You can still use Jessie and backports you just have to change a couple of things.

I take no credit for this I found the fix online and am using it on my Jessie box.

Change your sources to:

You can add contrib and non-free if you want

Then run:

echo "Acquire::Check-Valid-Until false;" | sudo tee -a /etc/apt/apt.conf.d/10-nocheckvalid

Jessie sources will still work.

Now you are running Devuan which should not have been affected by the change to the best of my knowlege.

According to:

You need to add the following

deb-src http://deb.devuan.org/merged jessie-backports main

In both cases you can install from backports with:

apt-get install -t jessie-backports [name-of-package]