Does anyone know what the meaning of "ärmelschonend" is? I assume it is some kind of "Redewendung", but I'm not sure.
You are correct in assuming that this is a colloquial figure of speech.
It is used to denote a type of work which is not physically exhausting, but rather work in an office. I would say, that it has a (slightly) pejorative tone. I think so because of the following semantical associations:
Ärmelschoner are a piece of clothing worn by clerks, to protect the sleeves from being polluted with ink (see https://de.wikipedia.org/wiki/%C3%84rmelschoner). So, I think, ärmelschonend has the subtext of addressing that the work is not exhaustive, similar to Schreibtischtäter, which also has a pejorative connotation (even when it is not used do denote officials in the 3rd Reich).
Also there is semantical link to the colloquial figure of speech sich nicht die Hände schmutzig machen (to not pollute one's hands with dirt), which also refers to "someone else is doing the real work for you". So, ärmelschonende Arbeit is a type of work where someone ist not polluting their sleeves / their hands with dirt.
All these critical, if not pejorative connotations stem from the fact that, in ärmelschonende Arbeit, "schonen" (preserve, spare, prevent from damage) stands in a contrast to the noun "Arbeit" (work).