Necessity is one of the trickiest terms in Catholic theology, as several different types of necessity can be distinguished, e.g. of means vs. of precept, absolute vs. relative, etc.
It's my understanding that nearly all theologians hold that the Sacrament of Baptism is absolutely necessary by necessity of means. Theologians hold that Holy Communion is necessary by necessity of precept and of moral necessity.
Question, in the context of BoD, is whether this necessity is satisfied in a BoD scenario, i.e. where the Sacrament of Baptism remains necessary even in BoD, i.e. where you could not have BoD WITHOUT the Sacrament of Baptism. St. Robert Bellarmine, understanding that the importance of retaining the necessity of the Sacrament, actually stated that they received the Sacrament of Baptism in voto.
This is where I disagree with the Dimond Brothers, where they hold that all BoD necessarily undermines the necessity of the Sacrament. You cannot have a Desire for Baptism without there being Baptism, so one could argue that it remains necessary for salvation even in a BoD scenario. Of course, as Father Feeney points out, the famous passage about votum refers to justification and not to salvation.
Now, BoD has become so expanded that there need not be any kind of explicit intention to receive Baptism to be saved by BoD ... according to many/most of its proponents. I'm not sure how one salvages the necessity of the Sacrament for Baptism when one claims that perfect contrition and charity along suffice, without any reference or thought of Baptism ... except for as some vague mystical "anonymous" instrumental causality, which I find to be borderline absurd.
In any case, we're not going to resolve this here ... until the Church's Magisterium intervenes.