I just noticed this question here on Meta that was migrated from Stack Overflow one hour ago but originally posted 64 days ago:
Questions older than 60 days can't be migrated, even by diamond moderators.
I just checked on some old question, and in the close dialog there's no option to migrate indeed:
I have also checked with questions 61 and 62 days old - no "This question belongs on another site in the Stack Exchange network" option, so this part is working well.
This leads to the conclusion that when there are pending migration close votes (cast when the question was less than 60 days old) the "This question belongs on another site in the Stack Exchange network" option still show up and such question can be migrated even after 60 days.
While it might be useful, it looks more like an overlook in the mechanism, so can this be fixed?