I have tried doing that and I was able to take a transactional log backup only using the option with with truncate only.
Backup log with truncate only means truncate the log and don't write a backup file out.
No matter what recovery model the DB is in, that command will never write a backup file out and, on full/bulk logged, it will break the recovery chain preventing further log backups until a full backup is taken.
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: Discussions on DB performance with occasional diversions into recoverability
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