Is it permissible to open a Twitter account for a deceased person, with the aim of having the reward of supplications and pleas for forgiveness written for them reach them?

The reward for acts of worship reaches the deceased, even if they did not perform them before their death. And the work of Dawah (Islamic propagation) pages is a righteous deed with ongoing reward, so there is no harm in creating a Dawah page for the deceased to receive its reward. There is nothing to prevent the creation of a page in the name of the deceased to pray for mercy upon them and seek forgiveness for them; the default rule for this is permissibility. Indeed, the Companions would pray for mercy upon the deceased when some of their virtues were mentioned.