For those who missed it, the original post did say
And of course, this is fake news. ISU, to my knowledge, are NOT doing this.
This thread is a hypothetical question posed by the OP, not a report of anything the ISU has actually considered.
Theatre on Ice competitions include a segment called Choreographic Exercise in which all skaters must wear plain black outfits. Here is an example of some:
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These are not required costumes, but rather choices by each team about what to wear within the strict limitations of what they are allowed to wear for this competition segment. It's not "mandatory costumes" in the sense that everyone has to wear the exact same thing.
Some teams choose to have the women wear black tights with black skating dresses (or leotards and separate skirts, or separate shirts and skating skirts). Some have the women wear leggings and shirts. Or unitards with or without skirts. Or shirts and pants. The men always wear the latter.
Some have long sleeves and turtlenecks. Some have 3/4 length sleeves, some short sleeves, some sleeveless -- with a variety of necklines.
The teams don't necessary train in these competition outfits. But whether they choose a more covered or less covered design, heavier or lighter fabric, might be influenced by the temperatures at their training rinks.
I can't imagine that the ISU designing one set of clothing for women and one for men and requiring all skaters in solo and couple disciplines to wear the exact same same outfits as their competitors, regardless of body type or preference for feeling warm or cool etc.
I could certainly imagine restricting clothing options for a technical program to solid colors without ornamentation (may not required to be black, and probably not forbidding lighter colored tights).
Less likely for rhythm dances where the clothing is expected to reflect the selected dances as traditionally performed off ice. And less likely for free programs where individual expression of a choreographic theme would be more encouraged.
The current rules say "the clothing of the Competitors must be modest, dignified and appropriate for athletic competition – not garish or theatrical in design. Clothing may, however, reflect the character of the music chosen. The clothing must not give the effect of excessive nudity inappropriate for the discipline. Men must wear full length trousers."
In the past, skirts have been required for women and sleeves for men.
I could imagine them getting stricter about imposing more restrictions to eliminate or restrict the use of sequins/crystals, illusion fabric, drapy material other than simple skirts, etc., in attempt to shift the the traditions more toward athleticism and less toward theatricality.
But in general, the ISU has never explicitly encouraged skaters toward theatrical costumes of any kind -- the rules have only forbidden or permitted various kinds of creativity that the skaters themselves (and designers they hire) come up with.
Skaters choosing to display wealth or the appearance of wealth through what they wear at competitions has been a part of skating culture at least since the fur-trimmed costumes in the days of outdoor rinks and furrier's daughter Sonja Henie as the one to beat.
Many skaters will try to one-up each other in having what they consider the most beautiful or striking costumes, and many skaters will try to express visual creativity and/or to express artistic creativity in relation to program themes.
At lower levels, some skaters may enjoy the idea of creativity but don't have the skating skills to show as much creativity with on-ice skills.
I guess the question is whether as fans we prefer to enjoy and encourage the skater-driven costume creativity aspect of skating culture, or whether we would prefer to see it reined in with focus more on the technical skills.
Personally, I am torn. I wouldn't mind having one competition phase or separate discipline that focuses primarily on technical skills with as little distraction as possible from other sources such as clothing, and another that focuses on a total artistic package where creative use of skating skills remains primary but supported by freer rein in music and costume choices.