Eric Sunshine [off-list ref] writes:
When I was pondering the issue before writing my original response,
two thoughts came to mind. (1) "git worktree add --force --orphan
<branch>" would be one way to make your case work; (2) given how
infrequently --orphan is used, we just punt and require people to
first use "git branch -D <branch>" if necessary (which has been the
status-quo for git-branch and git-switch).
FWIW, as I personally view that branch -d/-D, checkout -b/-B, and
switch -c/-C were all mistakes (they should have been -d, -b and -c
with and without --force, respectively), I find the combination of
"--force --orphan" a reasonable way forward.