From the developer on June 6:
"
Michael_Tsai Developer
SpamSieve 2.9.52 is incompatible with Apple Mail on the beta version of macOS 14. Although Apple has not made a formal announcement, it appears that Sonoma removes support for Mail plug-ins, perhaps because the mechanism was being used to
sideload iOS apps 21.
<snip>
Switching SpamSieve to use a Mail Extension requires major changes throughout the app, so this will be part of a major upgrade (version 3.0) rather than a maintenance update (like with
previous OS updates 1). This new version is currently in development, and I plan to roll it out to a private beta group soon and eventually to a public beta this summer, with the public release before macOS 14 ships in the fall."
My 2017 iMac won't be compatible with Sonoma, but I'll be replacing it with a new MacBook Pro in January when the new ones come out. So I have time for it to be updated.