When I hear towncrier I cry " @mauritsvanrees " right away. I'm only using manually tweaked changelog entries, Maurits knows more about towncrier I believe.

I tried it manually on a test package that does not use towncrier, with gh release create --notes-from-tag v3.0.0 and got this:

https://github.com/mauritsvanrees/mauritstestpackage/releases/tag/v3.0.0

The release note is "Tagging 3.0.0", which is from the tag, so not from the commit message.

I tried a change in zest.releaser which would work without towncrier as well:

$ git diff
diff --git a/zest/releaser/release.py b/zest/releaser/release.py
index cb60697..474bc6b 100644
--- a/zest/releaser/release.py
+++ b/zest/releaser/release.py
@@ -94,6 +94,8 @@ class Releaser(baserelease.Basereleaser):
         )
         self.data["tag-signing"] = self.zest_releaser_config.tag_signing()
         self.data["tag_already_exists"] = self.vcs.tag_exists(tag)
+        self._grab_history()
+        self.data["tag-message"] = self.data["history_last_release"]
 
     def execute(self):
         """Do the actual releasing"""

With that I released v3.0.4.
Ugly, but that could probably be worked around.

@mauritsvanrees so, would you suggest that the key to use should be tag-message ? 👍🏾 I'm happy with any name, as long as we can get this somehow merged 😄

Yes, the tag-message key would seem to work best.
This would be done by adapting your PR collective/zestreleaser.towncrier#26, which would mean no changes in zest.releaser itself are needed, if I see it correctly.