What a strange state of affairs that in 2020 an USP for an issue tracker is that it is fast. An issue tracker. How did we get here?
A simple glance at their website clearly shows to me that it's a tool to manage customer-facing ticketing mainly (allowing integration via email addresses). Linear's purpose is for managing software projects and features/issues within those projects.
I'm by no means a Linear advocate, I couldn't care less what people use to manage their software projects. But tools tend to have specific purposes and RT seems quite different from Linear with respect to their distinct purposes.