Most manufacturers approve their mastics for shower walls but not for shower floors. But that's the manufacturer.

My preference, as well as the preference of most others on this forum, is that while mastic is approved by the manufacturer for use in shower wall applications, I prefer to use thinset.

Understand that mastics are organic adhesives. Mastic simply dries out and hardens. It undergoes a physical change, which can easily be reversed, versus a chemical change, which cannot. When exposed to water or a moist environment for lengthy periods of time, mastic could absorb water and re-emulsify; it will soften. The bond between tile and substrate can be lost. That's why it's approved by the manufacturer for shower walls which get intermittent wetting, versus shower floors which can have standing water.

As others have written, thinsets are inorganic, they are portland cement based products. Portland cement undergoes a chemical cure when mixed with water. It doesn't simply dry out. It undergoes hydration, a chemical reaction, and becomes something different. The water in the portland cement based mix is "chemically consumed" in the reaction. It doesn't simply "evaporate away" like the drying process that mastic undergoes. Once thinset cures, it is what it is. Wet it, dry it, soak it, dry it, it's still rock hard and the bond between tile and substrate remains intact through repeated wet/dry cycles.

Soak a ball of cured thinset in water, it'll be wet, but it'll remain rock hard. Soak a ball of dried mastic in water, it'll turn to mush.

Anything "premixed" and "ready to be applied" that is sold in a resealable plastic bucket? No matter what the label says, it's organic mastic. It is not a "thinset" even if labeled so. It can absorb water and re-emulsify.

This is not a thinset. It is not a mortar. It is an organic adhesive:

Manufacturers have obliterated the meaning of "thinset". Technically "thinset" refers to an inorganic cement-based product designed to be used in a "thin layer" to set tile.

"Thinset" comes in dry powder form.

"Mastic" comes premixed in resealable buckets.

Here's a tidbit from REC regarding their product. Note that is is not waterproof. It's waterresistant.

So what to do, what to do...

If they used the mastic on the tiled shower floor, that's a no-no.

Mastic on the shower walls of an occasional-use residential shower (versus an often-used commercial shower, like a gym)? It's considered acceptable by some and by the manufacturer...but many consider it to not be a best building practice.

Mastic anywhere in a steam shower? No way.

My personal opinions regarding its use:
-not in showers
-not in tub surrounds
-not on floors
-not on countertops
-not for porous stone due to staining/bleed through issues
-yes to decorative (no moisture) installations
-yes to decorative backsplashes

Blah. Information overload. Good luck with your project.