We have usage stats to inform us about the usage of components such as the Legacy Particle System, so we don't need to speculate. Are you using Legacy Particles in your project(s)? Projects that will ship on a version later than 2018.2? (Which is the earliest conceivable version for total removal, if only script support disappears in 2018.1). If you think you meet those criteria, and want to give us information about your situation, e.g. how many hours you think you may lose performing a manual upgrade, we can take that kind of information into account.