[nkrisc]

I don't know if it's what the OP meant, but on macOS you can hold a letter key, and it will pop up a little dialog with a bunch of similar characters (hold 'n' and chose from 'n' with various diacritical marks), and you can just press the corresponding number key to choose it. I'ts not as fast as a dedicated key shortcut if you're constantly using the same character, but it's far easier if you're only occasionally using various characters and don't want to remember different shortcuts.

[bscphil]

Just tested this - it seems like a handy feature but apparently also dependent on the application using a specific textbox from the macOS UI toolkit. So it doesn't work in iTerm, and probably not in Firefox either? So not an "OS" feature exactly, and probably something the GTK folks could provide an equivalent for if they don't already.

[SyneRyder]

On macOS you can also type things like Opt-U to get an umlaut, followed by typing the letter you want underneath it (eg for ü, ë, ä, ö). ß is Opt-S, I think é is Opt-E E, etc. I prefer that to the AltGr+key shortcuts, which I find harder to remember. I don't use the Mac as my primary OS anymore, but that's one of the things I miss the most.

[lobo_tuerto]

On Linux there are dedicated combination keys to put accents on characters like: ä, á, é, ë, etc.

This is how it has always worked (I use a Latin-American keyboard layout). And I think it's the best of both worlds.

If you wanted and your font supports it you could have: ṕẃéŕý (accent key + character key).

All above without touching the AltGr key.