It's a phrase that I've usually encountered in the past tense. Google Ngrams shows that in 1800 "held forth" was used about three times as frequently as "hold forth" [1], but this ratio decreased as both uses became less common.

[1]: https://books.google.com/ngrams/graph?content=held+forth%2Ch...

I don't know a lot about the ngram corpus, but assuming there's lots of fiction everything will be skewed toward the past tense.