As I have described at Different font depending on file extension
it is possible to enable the OEM character set just for files of a specific type (= specific file extension).
By default there is only Default
in the drop down list in the configuration dialog Word Wrap/Tab Settings
and therefore the font settings and the setting OEM Character Set which is by default not enabled are for all files. If you add to the list of extension new entries, you can specify the font and also the setting OEM Character Set individually for all files with the extension listed for the current extension list entry. In the INI this is saved for example with following for 3rd list entry:
File Extensions2=bat nfo
For this file all files with the extension bat or nfo are viewed/edited with OEM Character Set while all other are edited/viewed with ANSI Character Set.
It looks like IDM has removed the command OEM Character Set from the menus and toolbars as also from available keyboard commands because most users don't need to edit files with old OEM character set and it was from time to time enabled by mistake. The INI settings still exist and advanced users can therefore still enable the OEM character set editing/viewing with keyboard conversion for specific files. And the ID for the command is also still recognized for a customized menu or toolbar.
An alternate method is to use File - Conversions - OEM to ANSI
after loading a file with OEM character set, and use File - Conversions - ANSI to OEM
before saving and closing the file. That method has the advantage to be able to copy text from / paste text into the file via the Windows clipboard in ANSI.
But back to your question: It now should be gone forever. Best remove the command from the menu to not enable it again by mistake.