some issues so far with MATE 1.21 ...
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mate-control-center doesn't start :

~ mate-control-center 
mate-control-center: symbol lookup error: /usr/lib/x86_64-linux-gnu/libmate-slab.so.0: undefined symbol: matemenu_tree_new

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mate-screensaver, similar error :

mate-screensaver-preferences: symbol lookup error: mate-screensaver-preferences: undefined symbol: matemenu_tree_new

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mate-power-manager also suffers from the same error as the devuan packages. systray icon broken (triangle with exclamation mark) and no power events recognised. suspend only works from menu, laptop lid closing doesn't trigger suspend as it should.
pinning and downgrading mate-power-manager & mate-power-manager-common to version 1.20.0-1+hezeh1 from hezeh.org/ascii and everything there works again.
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clock applet was missing after 1st reboot, but came back after a 2nd one. (?)
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mate-notification-daemon only works with "standard theme". all other themes end in error :

Error while displaying notification:
GDBus.Error.org.freedesktop.DBus.Error.NoReply: Message recient disconnected from message bus without replying

also dmesg error :

mate-notificati[4273]: segfault at ee ip 00007fd6163e0b4f sp 00007ffc261d0a10 error 4 in libgdk-3.so.0.2402.0[7fd6163a9000+78000]

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thanks,

using beowulf/ceres with MATE + openrc, fully upgraded from a fresh ascii installation with netboot iso.

can confirm the fix for policykit as Mithrandir writes.
similar apt preferences file, just some extra packages from hezeh.org repo, specialized for MATE power issues :

Package: policykit-1
Pin: version 0.105-18*
Pin-Priority: 1001

Package: libpolkit-agent-1-0
Pin: version 0.105-18*
Pin-Priority: 1001

Package: libpolkit-backend-1-0
Pin: version 0.105-18*
Pin-Priority: 1001

Package: libpolkit-backend-elogind-1-0
Pin: version 0.105-18*
Pin-Priority: 1001

Package: libpolkit-gobject-1-0
Pin: version 0.105-18*
Pin-Priority: 1001

Package: libpolkit-gobject-elogind-1-0
Pin: version 0.105-18*
Pin-Priority: 1001

Package: gir1.2-polkit-1.0
Pin: version 0.105-18*
Pin-Priority: 1001

Package: mate-power-manager
Pin: origin "www.hezeh.org"
Pin-Priority: 1001

Package: mate-power-manager-common
Pin: origin "www.hezeh.org"
Pin-Priority: 1001

thanks to all devuan devs for this great grassroots distro

tried upgrading to newest mate-power-manager 1.20.3 from ceres, but same errors (broken systray icon, no power events recognised).
so, back to hezeh.org version of mate-power-manager.
all other mate parts fully upgraded from beowulf/ceres.

mate-power-manager from ascii/ascii-backports/beowulf still suffers from this : https://dev1galaxy.org/viewtopic.php?id=601
installed just mate-power-manager from antofox repo and icon is displaying normally.