By default. Press left arrow to get them back. Or use start moar with --no-linenumbers=false.
man starts by checking the terminal width, and then formats man pages to fit that width.
Then, if moar adds line numbers, the rightmost part of the man page will be shifted out of view.
So we try to detect showing man pages, and in that case disable line numbers so that the rightmost part of the page is visible by default.