According to the documentation creating a file ~/.config/autostart/gnome-keyring-secrets.desktop with contents Hidden=true would also be sufficient, but this did not work during our tests.
Have you tried with just "Hidden=true" or also with the section header? Because the following is working for me:
[user@vault ~]$ cat .config/autostart/gnome-keyring-secrets.desktop
[Desktop Entry]
Hidden=true
[user@vault ~]$
Have you tried with just "Hidden=true" or also with the section header? Because the following is working for me: