I have found a number of discussions about this, for example
https://ubuntuforums.org/showthread.php?t=2222085 and
https://askubuntu.com/questions/98323/h ... th-lightdm
I have tried everything suggested there, except disabling accountsservice, which I am reluctant to do unless I can be assured that would be safe. Is that really necessary, and more importantly is it really OK to do? I see a lot of mate packages depend on the accountsservice package.
Anyway, users with UID < 100 in the mate login screen remain hidden using the following relevant sections of all the relevant configuration files mentioned in the solutions in the above links:
/etc/login.defs:
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#
# Min/max values for automatic uid selection in useradd
#
UID_MIN 500
UID_MAX 399999
# System accounts
#SYS_UID_MIN 100
#SYS_UID_MAX 999
#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN 500
GID_MAX 399999
# System accounts
#SYS_GID_MIN 100
#SYS_GID_MAX 999
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#
# User accounts configuration
#
# NOTE: If you have AccountsService installed on your system, then LightDM will
# use this instead and these settings will be ignored
#
# minimum-uid = Minimum UID required to be shown in greeter
# hidden-users = Users that are not shown to the user
# hidden-shells = Shells that indicate a user cannot login
#
[UserList]
minimum-uid=500
hidden-users=nobody nobody4 noaccess
hidden-shells=/bin/false /usr/sbin/nologin
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# FIRST_SYSTEM_[GU]ID to LAST_SYSTEM_[GU]ID inclusive is the range for UIDs
# for dynamically allocated administrative and system accounts/groups.
# Please note that system software, such as the users allocated by the base-passwd
# package, may assume that UIDs less than 100 are unallocated.
FIRST_SYSTEM_UID=100
LAST_SYSTEM_UID=499
FIRST_SYSTEM_GID=100
LAST_SYSTEM_GID=499
# FIRST_[GU]ID to LAST_[GU]ID inclusive is the range of UIDs of dynamically
# allocated user accounts/groups.
FIRST_UID=500
LAST_UID=399999
FIRST_GID=500
LAST_GID=59999
# The USERGROUPS variable can be either "yes" or "no". If "yes" each
# created user will be given their own group to use as a default. If
# "no", each created user will be placed in the group whose gid is
# USERS_GID (see below).
USERGROUPS=yes
# If USERGROUPS is "no", then USERS_GID should be the GID of the group
# `users' (or the equivalent group) on your system.
USERS_GID=100