I upgraded all of my machines and vms to Daedalus earlier in the week. My man outbound mail relay & secondary server had "backports" enabled inadvertently and exim broke in non-obvious ways because I have some custom routers I wrote about 10 years ago.
When I say non obvious, exim was running but rejecting any mail that relied on these routers. Thankfully as a secondary I just blocked inbound at the firewall until I can fix this mess.

Relevant message excerpt from the Exim-users list from about 4 years ago :

=======
As many of you may have noticed, with the release of 4.94 we introduced
strict checks for the data Exim uses in expansions. This broke old
configurations that used "tainted" data.

Unfortunately the introduction of these taint checks wasn't communicated
very well, and as not all of you were able to test the release
candidates, we understand that this "breaking" change was unexpected to
a majority of our user base. (Or will be, in case of Debian, which
currently ships 4.92, but having 4.94 already in its backports.)

The traffic on the mailing lists indicated that there are issues with
these taint checks. A good share of the issues was caused by broken
builds. But another share of the issues arose due to suddenly broken
configurations.
======

So if you use exim and have any custom lookups, please take note.
Fixing this is entirely non-obvious and poorly documented.

Just a note while I'm here :

Most of these machines were running Beowulf. Trial upgrades (on test systems) directly from Beowulf to Daedalus failed in horrible ways early on with a broken libc (missing libcrypt.so.1 and that was all she wrote).
The most expedient workaround for me was going Beowulf->Chimaera->Daedalus. If I did the upgrade in 2 steps I encountered zero issues (except the one noted above, but that's not a Devuan issue)
As I was upgrading some headless boxes on the other side of the world, I opted for "slow and steady wins the race".

Regards,
Brad