Hello @it's safe
The new iconography will not be available to apply to Windows 10 21H2
win10 2004, 20H2, 21H1, 21H2. they are just win10 versions by feature update released twice a year in SAC( semi-annual channel)
If 21H2 don't add any theme packs related to win11's rounded corner, frosted glass to its personalize→theme, then it is definitely not supported.
Here is the link for your reference to better catch the feature update.
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