Sounds like textbook Montanism to me.

The Reformers, and Puritans such as John Owen, were pretty clear that to claim special revelation outside of Scripture constitutes a denial of the sufficiency of Scripture.

I'm not sure whether I'd consider it damning heresy, but I think it is close, at best.

There are groups such as the Sovereign Grace movement that try to embrace both "special revelation" and Reformed soteriology . . . I do think it grossly undermines the authority of Scripture, and tends to have corresponding fruits. For instance, in my experience, people can practice all kinds of lawlessness by claiming that "God told me to." "God told me women can preach and lead men." "God told me that we owe reparations to those who were oppressed in the past." "God told me that we should keep our 'church' closed until the 'pandemic' is over." "God told me you should go out with me." "God told me this or that or some other absurd nonsense that is at least outside of, if not in direct conflict with, Scripture." And a lot of these folks also don't even bother to learn Scripture. Why, if we can just let "God" speak to us through our thoughts or through funny voices inside our heads?

What results is not Christianity, and it is not honoring to Christ, and it is indeed a step below Pentecostalism and Charismaticism, in that, at least in theory, the latter two groups do at least claim that "special revelation" ought to be checked against Scripture. Today's Montanists are led about by voices in their head, and in most cases, (provably if those voices contradict Scripture), those voices are not from God, but rather from the world, the flesh, and the enemy.

Aspiring student of Christ