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Ubuntu 16.04 (xenial) reached end-of-standard-support on April 29, 2021.
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It seems like this is by design, the workaround is to install xserver-
xorg-legacy.
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=802544
And
http://sources.debian.net/src/xorg-server/2:1.19.3-1/debian/xserver-xorg-core.NEWS/?hl=15#L15
** Bug watch added: Debian Bug tracker #802544
