Xindy compiled with an older version of libncurses
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jschrod
$ xindy --version
xindy release: 2.5.1
xindy script version: 1.18
xindy kernel version: 3.0
CLISP version 2.49.92 (2018-02-18) (built on bulldog [127.0.0.1])
architecture: X86_64
In the release with TeXLive 2020, updated today 2020-07-09, xindy requires libcurses.o.5 . This is no longer standard in Linux Mint 20 (or probably in Ubuntu 20), which installs libcurses6. So xindy won't run until you have installed libncurses5 from the repository.