Please use latin2 or utf8 (inputenc) with magyar babel warning

Starting from the 2018-04-01 release, LaTeX directly supports UTF-8, so the instruction \usepackage[utf8]{inputenc} does nothing apart from loading inputenc. In particular, utf8.def is not loaded, which makes magyar.ldf that no support for UTF-8 exists.

This should be addressed by the maintainer of magyar.ldf.

In the meantime, you can avoid the spurious warning:

\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
%\usepackage[utf8]{inputenc} % not needed with LaTeX 2018-04-01 or later

\def\magyarOptions{defaults=hu-min,suggestions=no}
\usepackage[magyar]{babel}

\begin{document}

Árvíztűrő tükörfúrógép.

\end{document}

A different strategy is to keep the same setup as before, but adding a command at the beginning.

\UseRawInputEncoding
\documentclass[a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

\def\magyarOptions{defaults=hu-min}
\usepackage[magyar]{babel}

\begin{document}

Árvíztűrő tükörfúrógép.

\end{document}

The setting to \magyarOptions is recommended in the magyar.ldf file notes.

The newest magyar.ldf (as of this week) on CTAN doesn't display this warning anymore.

I can confirm that \usepackage[utf8]{inputenc} is a harmless no-op in new versions of latex, pdflatex and lualatex. (I've just tried it with TeX Live 2018 on Linux.)

\usepackage[utf8]{inputenc} makes a difference in old versions of latex and pdflatex: without it non-ASCII letters don't show up. (This happens in TeX Live 2016 and probably also in TeX Live 2017.)

My recommendation is to keep using \usepackage[utf8]{inputenc} for compatibility with older versions of latex and pdflatex.