documentclass[12pt]{beamer}
usetheme{Copenhagen}
title{The title of the paper}
author{XYZ}
date{today}
begin{document}
begin{frame}
titlepage
end{frame}
begin{frame}{Outline}
tableofcontents
end{frame}

setbeamertemplate{footline}{}
begin{frame}[standout]
Welcome!
end{frame}


begin{frame}{Methodology}
tableofcontents
end{frame}

begin{frame}[standout]
Thank you!
end{frame}
end{document}

documentclass[12pt]{beamer}
usetheme{Copenhagen}
% usepackage{etoolbox}

% BeforeBeginEnvironment{frame}{%
%   setbeamertemplate{footline}
% }

makeatletter
define@key{beamerframe}{standout}[true]{%
setbeamertemplate{footline}{}%
}
makeatother

title{The title of the paper}
author{XYZ}
date{today}


begin{document}
begin{frame}
titlepage
end{frame}
begin{frame}{Outline}
tableofcontents
end{frame}

% setbeamertemplate{footline}{}
begin{frame}[standout]
Welcome!
end{frame}


begin{frame}{Methodology}
tableofcontents
end{frame}

begin{frame}[standout]
Thank you!
end{frame}
end{document}