Are Twitter accounts with the names of deceased individuals, which publish supplications and remembrances, considered a form of waqf, and does the reward for reading these remembrances and supplications reach the deceased, even if it was not an اثر (legacy/trace) that they left themselves?

The prescribed supplications and ذكر (remembrance of Allah) are acts of devotion by which a Muslim draws closer [to Allah]. The offering of their reward to others reaches them, and it is not a condition for its validity that the deceased must have left a legacy. The deceased benefits from their own good deeds and from any good deed whose reward is offered to them. This scenario does not fall under the category of Waqf (endowment) but rather under the category of offering acts of devotion.