You can search Google Scholar with custom ranges. Theodore Kaczynski left academia in June 1969. He graduated Harvard as an undergraduate in 1962. Sources seem to say he was a quite promising student, and his employment history backs this up: he got jobs at not one but two prestigious, research heavy universities. Wikipedia list his last publication to come out as 1971. On Google Scholar, very few of his pieces are cited, the most I saw was five citations (including self-citations).

After his arrest, the LA Times interviewed an academic mathematician about Kaczynski's impact as a mathematician:

The field that Kaczynski worked in doesn't really exist today, mathematicians said. Most of its theories were proven in the 1960s, when Kaczynski worked in it.

"He probably would have gone on to some other area if he were to stay in mathematics," Rung said. "As you can imagine, there are not a thousand theorems to be proved about this stuff."

The consensus seems to be that he had promising start in a field that's now dead, that his work has not too much influence today, but that he was an usually gifted student who could have had a successful career as an academic mathematician.