Nicknames

Curious thought, some of my friends and I were having, and I am curious about your opinions?

How do y'all feel about about nicknames for characters, in period pieces, where the nicknames likely didn't exist yet?

Ex: bob for a guy named robert Bubbles for a girl named champagne Kate for Catherine Ted for Theodore Etc.

Some of my friends argue ardently that they would not have existed in "x" time period. While many argue, "it's fiction, who cares?"

Thoughts?