The class amsart.cls has been updated to version 2.20.6 released 2020/05/29, where basically the only change was to support the new Mathematics Subject Classification (MSC) issued in 2020.
Previously the code was
\newcommand*\subjclass[2][1991]{%
\def\@subjclass{#2}%
\@ifundefined{subjclassname@#1}{%
\ClassWarning{\@classname}{Unknown edition (#1) of Mathematics
Subject Classification; using '1991'.}%
}{%
\@xp\let\@xp\subjclassname\csname subjclassname@#1\endcsname
}%
}
\newcommand{\subjclassname}{%
\textup{1991} Mathematics Subject Classification}
\@xp\let\csname subjclassname@1991\endcsname \subjclassname
\@namedef{subjclassname@2000}{%
\textup{2000} Mathematics Subject Classification}
\@namedef{subjclassname@2010}{%
\textup{2010} Mathematics Subject Classification}
This means that only 1991, 2000 or 2010 were supported as optional arguments to \subjclass. The idea is to avoid “illegal” release dates of the MSC. The optional argument defaults to 1991, which is also the date used (after the warning message) if an unsupported date is used.
In the new release we have essentially the same, but with some small changes
\newcommand*\subjclass[2][2020]{%
\def\@subjclass{#2}%
\@ifundefined{subjclassname@#1}{%
\ClassWarning{\@classname}{Unknown edition (#1) of Mathematics
Subject Classification; using '2020'.}%
}{%
\@xp\let\@xp\subjclassname\csname subjclassname@#1\endcsname
}%
}
\@namedef{subjclassname@1991}{%
\textup{1991} Mathematics Subject Classification}
\@namedef{subjclassname@2000}{%
\textup{2000} Mathematics Subject Classification}
\@namedef{subjclassname@2010}{%
\textup{2010} Mathematics Subject Classification}
\@namedef{subjclassname@2020}{%
\textup{2020} Mathematics Subject Classification}
\@xp\let\@xp\subjclassname\csname subjclassname@2020\endcsname
Besides adding support for the 2020 edition of the MSC, this date has become the default, if no optional argument is used.
So you need to update your TeX system to TeX Live 2020 or use the workaround suggested by Tanvir.
With an up-to-date system, \subjclass{...} will produce 2020.
\maketitlebefore\end{document}I get 2020. You should update your TeX system. – egreg Sep 30 '20 at 15:58amsartpackages and documentclass. – Sep 30 '20 at 16:49