For me, it's really possible to have a best friend of the opposite sex and still have nothing going on between them. I think it depends on respect and knowing your limits. Just because two people are close doesn't mean they like each other in a romantic way. But I understand why people always talk about it. Sometimes, your real partner may feel uncomfortable or jealous if they don't fully understand the friendship. That's why it's important to be open, honest, and not hide anything. If your partner feels safe and knows everything, there should be no problem. Most problems happen when there are secrets or things that make others confused.
Indeed you are right, when one is into a relationship or when marriage is involved, and one still intend of maintaining their old relationship or friendship know as ' Bestfriends', then I don't see anything wrong or worth hiding it from their spouse's or their partners, they should know the truth to avoid future problems or misunderstandings, which might lead to destruction, which will not result in good seed. So its better, wise and beneficial not to hide the truth especially to those that deserve or that matters.