I have a fair few concerns about this proposal, I see that a few of them have been addressed since the first recommendation, the concept of there being such a thing as 'communist architecture' is something that would require a fair bit of grappling.
In terms of the big Communist memorials then perhaps there is something to be said in its favour, as these are essentially what triumphal arches were in the roman world. The Mother Russia memorial in Vologograd nee Stalingrad would probably be the pick of the bunch.
There are also the large scale Monumental Palaces of the People, To mind the best examples (shamefully this is mostly limited to Europe due to my lack of knowledge of communist planning in other continents) of these buildings would be The Palace of Arts in Warsaw and the Palace of the People in Bucharest (I may be wrong with the precise names). I would certainly not recommend Bucharest be put forward and would doubt the merits of the Warsaw case being in the 'Top 50 missing sites'.
In terms of schools of architecture that have a communist heritage then the main examples would be the Russian Constructivist (Futurist) school, the most famous examples being Rusakov Workers Club, Shukhov Tower and the Narkonfin Building of which the latter two are in danger of being ripped down and especially in the case the Narkonfin Building UNESCO would jump at the opportunity to protect it, but I would be hesitant to put these forward as part of the top 50.
In terms of town planning a lot of the aspects of the ground works are derived from 'non-communist sources' perhaps broadly designated as socialist or egalitarian schools. This would incorporate things such as the Bauhaus but more importantly the Berlin housing estates, which are currently on the tentative list. Similarly the industry/ housing relationship solutions are partially shown in the tentative Czech site of Ostrava but again neither of these are good enough to be in the 'top 50'.
As for Karl Marx Allee I really wouldn't say this was really worthy for consideration, despite the Stalinist - Baroque styling and I wouldn't put it as part of the 50 most important sites not on the list.