In Animal Farm, what is forbidden shortly after the executions?

The singing of the anthem which Old Major had taught them, Beasts of England, was abolished.   

After the executions, the distraught and frightened animals had gathered on a knoll overlooking the farm. Clover, especially, was saddened by what had just happened and felt that that had not been what they had worked so hard for. They had now come to a time where they were too afraid to speak up and felt threatened. Because...

The singing of the anthem which Old Major had taught them, Beasts of England, was abolished.   

After the executions, the distraught and frightened animals had gathered on a knoll overlooking the farm. Clover, especially, was saddened by what had just happened and felt that that had not been what they had worked so hard for. They had now come to a time where they were too afraid to speak up and felt threatened. Because she could not express what she felt in words, she began singing Beasts of England. The other animals took up her lead and mournfully and slowly sang the tune three times over, in a manner which they had never sung it before.   

It was at this point that Squealer, accompanied by two dogs, approached them and announced that the anthem was, by order of a special decree from Napoleon himself, heretofore banished. It was forbidden to sing it. When Muriel asked why Squealer explained that the anthem had served its purpose and was no longer necessary. The anthem was a song of the Rebellion and the Rebellion had been completed. The enemies of the animals, both within and without, had been overcome and the executions were the final act of the Rebellion.

Squealer explained, furthermore, that in Beasts of England the animals expressed their desire for a better society, free of man's tyranny, and that society had now been established. The song, therefore, had no more relevance. Some of the animals might have protested at this point but were prevented from doing so because the sheep loudly started bleating 'Four legs good, two legs bad,' which put an end to any further discussion. 

After this, Beasts of England was never heard again. In its place, Minimus the poet had composed a replacement which never sounded as good as the original anthem. 

The pigs under Napoleon's leadership had now reached a point where their dominance was unquestionable. Napoleon has become a tyrant and he would use his power to further oppress his own kind and ensure privileges for himself and the other pigs exclusively. At this point, he was comfortable in the knowledge that all his decisions, no matter how unfair or harsh they might be, would be executed without any resistance - the executions made sure of that.