It looks like, for 2.9, the Merge shortcut is M for everyone. In my case it is AltM, so I must have set a new shortcut. I've checked the Preferences for the currently selected keymap setting, and here is what I see (image below). As you can see there is no Restore button that would allow me to go back to M. Souldn't there be one? If I change it to M the Restore button will appear (but to go back to AltM!). If I switch to one of the default keymaps in the top dropdown list I can find a setup where M is indeed the default setting. So I must be missing something here in the Preferences management.
Edit: So it looks like exporting and reimporting my own custom keymap (whatevername.py file) makes the new keymap its own default. Once this keymap is loaded there's no Restore button available to go back to some factory settings. If you create a new shortcut within this setting then the Restore button will appear, but to go back to this keymap default setting. I find it weird, there should be a way to go back to a default 2.9x keymap setting.