I have, for a variety of reasons. Not all relationships referenced here were romantic, some were platonic.

Some alters felt so endangered, cutting the person out of my life had to be immediate, and the threatened alters didn't feel safe saying goodbye. Either because they needed the relationship to be over NOW or because they feared that verbally ending the relationship would lead to more imminent danger.

There have been times where an alter that has a relationship with someone will suddenly go dormant, and the remaining alters either don't know the other person or don't think the relationship was significant enough to warrant a verbal closure, because they have no emotions invested in it.

It has happened due to intense anxiety. There's some kind of stress in the relationship- something needs to be discussed, or they're doing something that is triggering for us, or even totally unrelated stress from day-to-day life, that makes us feel incapable of carrying on interaction with others. With this one it's usually not intentional, but the more time that goes on, the more anxiety we feel about reaching out to them again, and eventually hit a point of no return where it's impossible to overcome the amount of anxiety generated by considering talking to the person.

Oh, another one is there have been times we've stopped talking to people because they associate with someone we don't want to be around, usually because at least one alter will feel like they are threatening or dangerous. So in our effort to get away from the person that's threatening, we'll cut out a lot of mutual friends as well.

For me it's never been for the purpose of keeping options open for later. Honestly it's probably impossible to know why they would do that without them explicitly telling you. Hell, they might have no idea why they're doing it. It could be literally anything and mean literally anything.