Oh, FFS...
FWIW, I literally just followed the instructions:
"Before GNU Mailman can unsubscribe you, you must first confirm your
request. You can do this by simply replying to this message."
So I hit reply and sent it without touching anything.  Oh well.
Maybe it's a problem with Mailman, maybe it's a problem with KMail, I dunno.
In any case I'll try the second method now.
Am Samstag, 19. Juli 2025, 16:29:31 Mitteleuropäische Sommerzeit schrieb Marc
Joliet:
  Am Samstag, 19. Juli 2025, 16:24:13 Mitteleuropäische
Sommerzeit schrieb
 linux-audio-dev-
 confirm+d11887e9e044134819e2315c29391879bc8438d5(a)lists.linuxaudio.org:
 > Email Address Unsubscription Confirmation
 >
 > Hello, this is the GNU Mailman server at 
lists.linuxaudio.org.
 >
 > We have received an unsubscription request for the email address
 >
 >
 >     marcec(a)gmx.de
 >
 >
 > Before GNU Mailman can unsubscribe you, you must first confirm your
 > request. 
 You can do this by simply replying to this message.
  >
 > Or you should include the following line -- and only the following
 > line -- in a message to linux-audio-dev-request(a)lists.linuxaudio.org:
 >
 >
 >     confirm d11887e9e044134819e2315c29391879bc8438d5
 >
 >
 > Note that simply sending a `reply' to this message should work from
 > most mail readers.
 >
 > If you do not wish to unsubscribe this email address, simply disregard
 > this 
 message.  If you think you are being maliciously unsubscribed from
   the list, or
have any other questions, you may contact
     linux-audio-dev-owner(a)lists.linuxaudio.org 
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