Transfer from Gare Est in Paris to Gare Montparnass

Not recently, but it is straight forward. Issue is that the Metro station is on the far side of the Tour Montparnasse from the train station, so it is a fair walk underground, part of which has a moving walkway (if it is working).

I think the 6 metro is closer to the Gare than some of the other lines. I know the 4 and the 12 are a long long walk away. You could take the 5 to Place d'Italie and change to the 6 there. It is a fairly long trip -- I'd guess 35 or 40 minutes.

Line 4 is currently closed south of Les Halles, until July 14th: beware.
Without it, the Est-Montparnasse transfer is frankly an ordeal: 5 to Italie, 6 to Montparnasse.