Ice cream was very popular in the 1920's, but it would usually be called "ice cream". There is some possibility that it would be truncated for space in a newspaper, or expanded to mean any frozen desert.

Most likely it meant what "ices" meant a hundred years prior and a hundred years later, though I don't know the exact that kind was popular just then. That is, "water ice", sorbet, etc. This was before home refrigerators, so would have had a fairly soft texture that could be kept from melting too quickly in an ice box. While these are usually sweetened, they could be served more like an appetizer, especially with a fruit flavor like peach or strawberry. And also so it wouldn't melt holding... :)

Ice cream was frequently referred to as ‘ices’ up to the early 20th century in Britain. eg We went into to town and bought ices.

I'm guessing it may have been sweet or savory - something that was served on ice, perhaps - sherbets and maybe iced things like cold seafood or tomato ices and aspics?

It's what we might call an amuse bouche.

I googled it and found some info...this is from a 2018 dinner: 4th course: The Ice course featured orange + herb salad with asian
nuances of peanut, coconut milk + exotic silk road spices and a
himalayan mist.
Ice course
A newspaper in Texas from 1910 refers to an ice course being served to ladies calling on a bride to be. Sounded like it was a HOT day and it was the only reference to food being served, so maybe could have been anything cold and refreshing?
In reading through this compilation, an ice course is referred to, but ice cream is referred to separately and most of the posts mention ice cream, so I don't think an ice course is ice cream.

I couldn't edit the above but found this from an Emily Post 1922 book on etiquette: "In the present world of fashion the “dessert” is ice cream, served in
one mold; not ices (a lot of little frozen images). And the refusal to
call the “sweets” at the end of the dinner, which certainly include ice
cream and cake, “dessert,” is at least not the interpretation of either
good usage or good society. In France, where the word “dessert”
originated, “ices” were set apart from dessert merely because French
chefs delight in designating each item of a meal as a separate course.
But chefs and cook-books notwithstanding, dessert means everything sweet
that comes at the end of a meal. And the great American dessert is ice
cream—or pie. Pie, however, is not a “company” dessert. Ice cream on the
other hand is the inevitable conclusion of a formal dinner. The fact
that the spoon which is double the size of a teaspoon is known as
nothing but a dessert spoon, is offered in further proof that “dessert”
is “spoon” and not “finger” food!"
This is an amazing read about giving formal dinners in that era: Emily Post on Formal Dinners

I would see an ice course as serving Italian ices which have been around a very long time.

In the 1920s and earlier I’m absolutely certain ’ices’ we're simply ice creams, sorbets or desserts made with them. Nothing fancier than that and not savoury.

I had the same thought as LoneJack.