Sample code:

a = f"{1[1 - 2 :]}"
b = 1[1 - 2 :]

When running with ruff --select E203 --preview, the first line triggers E203:

test.py:1:15: E203 [*] Whitespace before ':'
  |
1 | a = f"{1[1 - 2 :]}"
  |               ^ E203
  |
  = help: Remove whitespace before ':'

The same slice on the second line which is outside of a format string does not trigger E203.