In the 21w15a changelog, it says "Worlds created in any version between snapshot 21w06a and 21w14a can no longer be opened" implying that this doesn't include 21w06a and 21w14a. This could be corrected to "from" instead of "between."
Probably affects all launcher versions.
Can confirm. Same thing on https://www.minecraft.net/en-us/article/minecraft-snapshot-21w15a.