When I started studying English, more years ago than I care to count, I was taught that, as an adverb at the end of a sentence, "though" should be preceded by a comma. That's the way I have always written it. A current search in mainstream dictionaries, however, shows that my teacher was only partially right. Most sources give examples of this adverb preceded or not by a comma.
From Merriam-Webster we have the following examples, with and without a comma.
- "I know we didn't win, but I was happy with how we played, though."
- "It's hard work. I enjoy it though."
I've tried to find what n-gram shows, but the moment you add a comma you make it impossible to find such data.