What is the “issur” exactly of “uvdah d’chol” (and I’m not asking what does it mean.) Is it b’geder a din in “maris ayin” is it “stam a shvus” is a gezera?
Uvdin De-Chol is not related to maris ayin, nor to a specific decree. It is a rabbinic prohibition against activities that are considered weekday-like and therefore contravene the principle of shevisah (rest) on Shabbos.
The Ramban (Parashas Emor) explains (as Chazal note) that Shabbos must be distinct from the weekdays in all ways, and makes particular note of uvdin de-chol as activities that erode the distinction between them.
The Beis Yosef (328) notes that under extenuating circumstances (he notes a case of suffering) there is room to permit a prohibition of uvdin de-chol.