/bin/sh is linked to dash which is the default for sh in Ubuntu
The problem seems to come from the second part of that line where asdf.sh is sourced although using a dot here seems to be POSIX compliant. So I don’t think there is anything wrong with that line per se.
running sh ~/.asdf/asdf.sh gives a similar error to the one I observed earlier
/home/my_home/.asdf/asdf.sh: 8: /home/my_home/.asdf/asdf.sh: Bad substitution
So even though the asdf.sh has #!/usr/bin/env bash at the top it seems to be ignored because it is called via sh
