Hi,
since openbsd 7.9, filter-spamassassin-0.8 exits with an error due to
the protocol version bump.

Aug  8 19:49:33 base smtpd[47270]: info: OpenSMTPD 7.9.0 starting
Aug  8 19:49:33 base filter-spamassassin[55151]: start
Aug  8 19:49:33 base filter-spamassassin[55151]: register
Aug  8 19:51:39 base smtpd[13205]: 933726ef1793b5aa smtp connected
address=145.131.90.155 host=mx-out2.startmail.com
Aug  8 19:51:39 base filter-spamassassin[55151]: 933726ef1793b5aa
protocol report 0.8
Aug  8 19:51:39 base filter-spamassassin[55151]: exit
Aug  8 19:51:39 base smtpd[86530]: warn: lost processor: spamassassin
exited abnormally
Aug  8 19:51:39 base smtpd[86530]: Exiting

Line 214 of filter-spamassassin.go (downloaded from the website) expects
version 0.7:

if (t != "filter" && t != "report") || ver != "0.7" {

Changing only that line from 0.7 to 0.8 and rebuilding the executable
seems to make it work ok again. I will continue to monitor for the next
few days.

Thanks to Harald Dunkel's report to openbsd-misc in November 2023 when
the smtpd-filters version was bumped from 0.6 to 0.7.

The examples in the smtpd-filters man page show a version of 0.7.

kind regards
Richard