Refers to a past case or instance that serves as a reference, often in negative constructions like ためしがない (there is no precedent / it has never happened).
There is no precedent for snow falling this early.
He has never kept a promise.
He has never once come at the promised time.
例（れい）is the common, everyday reading meaning 'example' in general contexts, whereas 例（ためし）is a rarer reading restricted mainly to set phrases about past precedents.
Standard kanji spelling for this meaning.
Rare variant; 例 is strongly preferred in modern Japanese.
Irregular okurigana usage; not standard.
The reading ためし is a native Japanese word (kun'yomi) attached to the kanji 例, which carries the meaning of 'example' or 'precedent'.