Close messages without deleting the conversation in Big Sur

A more constructive and pro-active solution is to Product Feedback

RupertGiles wrote:

Prior to Big Sur, I could close a message by clicking the "X" in the upper right corner of that conversation in the messages list. The next time I sent a message to that person, or they sent one to me, the entire conversation would re-open.

Now, it seems that I can only DELETE a message entirely, which means that if I want to retain a conversation, I must leave it open in the Messages window. Why was this change made? Is it possible that this feature will be returned in a subsequent update?

Not to side step your two questions but Only Apple can possibly answer both your questions