Do you share your academic papers with your supervisor before sending them to journal editors for publication, asking your supervisor for a second opinion? Or simply send them to the editor without your supervisor having had a look first? What do you normally do?

I think it will vary a lot, depending on personalities, and also if your supervisor is to be a co-author. In humanities it's not normal for supervisors to be co-authors. In sciences it is. You certainly shouldn't submit a co-authored paper with your supervisor's name on it without letting him/her look at it before submission.

I published multiple journal papers as a history PhD student. I didn't send them to my supervisor first, and just got on with it. These were sole-authored papers.

My field is in the humanities. I am discussing sole-authored papers here. Any of you acknowledge the supervisor is he / she provides a second opinion? I think that this is a must.

I always acknowledge help I've received, usually in a footnote at the start. Most journal editors allow this type of thing. I always include the text for this as a note of thanks, to be included if the paper is accepted for publication.