You ask who notifies removed trustees of trust. I take it that the trust is a living trust, and that your step-mom is now the trustee of that trust and
that she has decided to remove some or all of the successor trustees.
I do not know of any statute that governs notifying a (successor) trustee of his or her removal. Normally the notification would be given, in writing, by the person who created the trust, the "grantor," who is also normally the trustee of that trust. So it seems that your step-mother is the person to give notice, if my assumptions about how the trust works are correct.
The removed trustees, or others, could possibly raised the legal issue of whether your step-mother was mentally competent at the time she removed the trustees. You state that she is in "the early stages of dementia." To me, that leaves it vague whether she retains the capacity to make competent mental decisions.
It could be wise for her to prepare a statement of why she removed the trustees, as evidence that she was competent and understood what she was doing.