Why is there no option for the term 'ritard'/'ritardando'?
It's nowhere to be found in any of the palettes/symbols under F2. Sure, you can type in your own text, but this seems like a glaring mistake for such a common term. It should have its own selection in the palette menu.
Comments
Your right, but unlike other terms in the lines palette., Rit has no effect on playback. You can easily make your own versions and save them for your own use. Just like everything else in the palettes.
It will be in MuseScore 4
Meanwhile, check out the excellent Tempo Changes plugin (see Download / Plugins in menu above), which lets you easily add these texts along with the necessary hidden tempo markings to get the desired playback effect.
For now, this is what the MS3 handbook has to say about it: https://musescore.org/en/handbook/3/tempo#ritardando-accelerando