I don't know why the footnote doesn't work. It does without \par.
\documentclass[a4paper, 12pt]{article}
\title{title%
\footnote{
para1\par
para2
}%
}
\author{author}
\begin{document}
\maketitle
text
\end{document}
For why it doesn't work see: Why don't we always use \long\def instead of \def?
So to fix it, you could redefine \title and \thanks from latex.ltx to be \long which will then allow the paragraph break.
\makeatletter
\long\def\title#1{\gdef\@title{#1}}
\long\def\thanks#1{%
\footnotemark
\protected@xdef\@thanks{%
\@thanks\protect\footnotetext[\the\c@footnote]{#1}}}
\makeatother
But an easier option is just to define your footnote text as a separate macro which gets around the error checking.
\documentclass[a4paper, 12pt]{article}
\def\titlefootnotetext{para1\par para2}
\title{title\thanks{\titlefootnotetext}}
\author{author}
\begin{document}
\maketitle
text
\end{document}
\def before.
– Max
Mar 16 '19 at 21:19
\parto have checks that often aren't appropriate in a footnote environment. Instead of\paryou can use\endgrafwhich does only the basic positioning, without the extra checks. – barbara beeton Mar 16 '19 at 22:17