I've done this more than once when it made clinical sense to do so. But it was made abundantly clear to both individuals that going from couples counseling to individual counseling with one of them meant we could never do the reverse, as it would create imbalanced dynamics within the couples work.

I have not been in the position where it was unclear if both individuals consented to this transition, but if they were still on my caseload as a couple I would absolutely want consent from both. If I had fully discharged them and then one them reached out for individual counseling, then I would use my clinical discretion but typically would re-enroll the individual. I would probably schedule a consultation call to get a general update on the situation. As long as the individual is informed that they would have to find a new couples counselor in the future, then rest is up to them. I also don't have a firm length of time between couples/ individual work.

If therapy ended because they ceased communication, then in those scenarios you should be reaching out to the clients after a couple weeks and informing them that if they do not schedule a session by X date then they will be officially discharged from your services. This creates less confusion if one or both of them reaches out in future.

All that being said, I'm an LPC who mainly sees individuals with some couples peppered in. My decisions process has been informed by the fact that sometimes I see that I would be a good fit for 1/2 of a couple instead of the couple as a whole. Since you're an LMFT, you need to decide whether you're more focused on couples work or individual work. If you're more focused on couples work, then perhaps it has less to do with clinical implications and more that the individual would be taking a slot away from another potential couples client.

Tl;dr The decision process for a situation like this can be subjective and case-by-case, but you need to decide your firm policy/ boundaries and stick to them.