According to the patch notes, you’re not supposed to be able to respec points that would leave you with orphaned passives at the top of a mastery tree that you wouldn’t have been able to qualify for normally:
You must meet the requirements for all nodes you have after the respec occurs (e.g. if you have points in a node that requires level 15 in a mastery then you must maintain at least 15 points allocated to nodes with a lower requirement in the same tree).
However, this restriction doesn’t appear to be working:
I just unspecced 10 points out of Volatile Blood. This should have made my points in Soul Maw illegal (I only have 45 points in Lich total, including the 5 in Soul Maw, but Soul Maw requires 45 points before you can spec its first point). Instead, I still have my 5 points in Soul Maw.
This is super abusable - you can take points at the bottom of a tree to qualify for (stronger) passives at the top of the tree, then unspec your low-tier passives, leaving only the strongest capstone passives in place.