Oracle doesn't treat Boolean expressions like other expressions — it handles them as syntax rather than as a type — so CASE
expressions can't evaluate to Booleans.
In your case, I think the clearest code is if you just rewrite it a bit:
AND ( TBL_DTL_HOST.FEATURE_ID = 'F020'
OR (i_Feature = 'All' AND TBL_DTL_HOST.FEATURE_ID = 'F021')
)