The old API keys had 32 hex characters after the prefix because that was what the UUID generator produced. That length wasn't a deliberate product choice.
I shortened newly created keys to 20 characters after the prefix. This shows the length of a secret key, with the actual characters replaced:
parse_xxxxxxxxxxxxxxxxxxxx
The body uses lowercase letters and digits, leaving out 0, 1, i, l, and o. That makes it easier to tell the characters apart when you're looking at a key on screen. Public keys keep their parse_public_ prefix and use the same length for the body.
Your existing keys still work. Both lengths are accepted, with the same X-API-Key or Authorization: Bearer header, so there's no need to replace a working key just to get a shorter one.
The next key you create in Keys uses the shorter format.