Apparently, while the _split_longopt function is gone from /usr/share/bash-completion/bash_completion, there are still lots of packages which rely on its existence:
$ grep -rl "\\b_split_longopt\\b" *
completions/opensc-asn1
completions/netkey-tool
completions/pkcs15-init
completions/opensc-notify
completions/eidenv
completions/dtrust-tool
completions/goid-tool
completions/opensc-tool
completions/npa-tool
completions/iasecc-tool
completions/grub
completions/pkcs11-register
completions/dnie-tool
completions/cmake
completions/cryptoflex-tool
completions/sc-hsm-tool
completions/westcos-tool
completions/pkcs11-tool
completions/egk-tool
completions/openpgp-tool
completions/piv-tool
completions/opensc-explorer
completions/pkcs15-tool
completions/cardos-tool
completions/gids-tool
completions/pkcs15-crypt
As @steeldriver notes, this is likely due to your using a testing version of a distribution, which is undergoing possibly-breaking changes. At some point before Debian Trixie / Devuan Excalibur is released, this should get fixed.
In the mean time... you could replace the function name in all those files, like so:
sudo sed -ri s'/_split_longopt/_comp__split_longopt/' \
$(grep -rl "\\b_split_longopt\\b" /usr/share/bash-completion)
/usr/share/bash-completion/bash_completion- have you modified that file?