I purchased an old farmsite and there are a number of trees appears over the years grew just around old metal posts. Am I best to remove these posts with saw/torch or leave them as is?

Do you plan to reuse the metal? Is there a tripping hazard likelihood?
Personally, I'd probably leave it be.

Alternatives include cutting off both ends of post with a sawzall...

Or cutting through overgrown lip of the tree with chainsaw...

Kind of dangerous to do the chainsaw work... but it is possible... Or... maybe just use a hand saw...

Not sure that it's worth doing though... Or whether the tree would survive the operation....
"The operation was a success, but the patient died."

Fascinating example!

I would remove them as it always will look like something you "have to" do, and it's better to get it done.

You have an old farm site. No reason to remove the posts entirely. As is, they are a conversation piece.
For safety reasons if you want, cut them way back to the tree bark and file the edges decreasing the chances of a sharp metal edge injury.
If it was my property, that's what I would do.

I have quite a few trees embracing posts. I gives them character.