Is it safe to cancel tmutil delete halfway through? I wanted to delete my oldest backups to free up space on my backup drive.

Due to some confusion, I "accidentally" deleted all backups.

The image shows the current progress of the command:



Is it safe to just ctrl-c/ctrl-d this?

I don't care about deleting the rest, I bet there is enough space by now and if not I'll find out and obviously delete again.
You have two choices: let it run and delete everything; or stop it and see what's left. The residual backup may be incomplete, in terms of files and times, but it will be something. 

My advice for TM has always been: don't mess with it. let it do its thing. When the drive gets full, either let it start deleting the oldest backups (which it will do itself), or get a bigger drive and start again or transfer the archive.