My understanding is that Fox has attempted many times via most of their hosts to get Biden and Harris on their shows, with no acceptance.
Remember, the same rule goes for MSNBC and CNN.
However, there is a difference, legally, between a news item and a campaign speech.
Any network or channel that carries a campaign speech all or in part may consider it to be a news item. I have seen Fox carry almost complete Biden appearances (as few as they have been) as news stories. Yes, they interrupt to comment or snark, but they carry some. In fact, I was looking for Bident's first speech out of his home a couple of weeks ago, and while neither of the Blue networks were carrying it, Fox was, although with in and out snippets.