Your MWE does not compile without errors, eg., for term_1
etc.
I don't understand what you mean by "header". I always think of "header" as applying to something at the top of each page; an extended version (more \lipsum
) of your MWE does not show any of these. If by "header" you mean the title of a section in the document then your \section{Intorduction}
produces one, but you say you don't want any headers. I am confused. I can't imagine a document without any such headers. If you don't want such things then don't use \section
etc.
If you just want to remove the title Index then change your \makeindex
to:
\makeindex[title=]
as indicated by the imakeidx
documentation.
The page geometry in a document is constant unless you change it, which your MWE does not do.
EDIT
After a comment by the OP @user136555 where the OP clarifies that by "header" is meant headers and footers produced by the likes of fancyhdr
I have no idea what the problem is. The OP's MWE only produces a single index page and therefore cannot show headers, or not, on subsequent pages. Below is an extended version of the OP's MWE which generates multiple Index pages.
% indexprob.tex SE 592445
\documentclass[oneside,a4paper,12pt]{report}
\usepackage{polyglossia}
\setmainlanguage{english}
\usepackage{microtype}
\usepackage[margin=1in]{geometry}
\usepackage{lipsum}
\renewcommand{\indexname}{}
\usepackage{imakeidx}
\makeindex%[title=]
\setlength{\textheight}{0.22\textheight}
\begin{document}
\tableofcontents
\section{Intorduction}
\lipsum[1-3]
%term_1\index{term_1}
%term_2\index{term_2}
%term_3\index{term_3}
%term_4\index{term_4}
term-1\index{term-1}
term-2\index{term-2}
term-3\index{term-3}
term-4\index{term-4}
\lipsum[1]
term-5\index{term-5}
term-6\index{term-6}
term-7\index{term-7}
term-8\index{term-8}
\lipsum[2]
term-9\index{term-9}
term-10\index{term-10}
term-11\index{term-11}
term-12\index{term-12}
term-13\index{term-13}
term-14\index{term-14}
term-15\index{term-15}
term-16\index{term-16}
term-17\index{term-17}
term-18\index{term-18}
term-19\index{term-19}
term-20\index{term-20}
term-21\index{term-21}
term-22\index{term-22}
term-23\index{term-23}
term-24\index{term-24}
term-25\index{term-25}
term-26\index{term-26}
term-27\index{term-27}
term-28\index{term-28}
\printindex
\end{document}
This produces 3 Index pages and no headers on any of them.
I worked hard on this producing the multiple index pages and am now rather unhappy. --- GOM