Can we use "setting" to mean "the end" of something?

Solution 1:

You could say something like "the sun has set on their friendship" as a slightly whimsical and poetic metaphor, once the situation is in the past. The use of the noun "setting" implies a prediction of something which will occur, and does not really work in a metaphorical sense.

Solution 2:

Following up on @Kate Bunting's comment, sunset is often used in government to describe laws that are intended to stay in effect for a certain period of time and then expire. For example: "The legislature imposed a surcharge on the corporate tax in year x that was scheduled to sunset in year y."

When used in this context, sunset can be used as a noun, an adjective, a transitive verb, or an intransitive verb.

• Noun. "Eliminates the sunset of the Municipal Conveyance Tax rate increases" - The Connecticut Office of Fiscal Analysis summary of Senate Bill 1239, An Act Concerning the Budget for the Biennium Ending June 30, 2013."
• Adjective. "There was no sunset provision striking it from the books after five years..." - CTMirror (a daily public policy online publication covering Connecticut state government).
• Transitive verb. "Public Act 19-117, signed by Gov. Lamont on June 26, 2019, sunsets the $250 biennial Connecticut Business Entity Tax (BET)." - Connecticut Department of Revenue Services
• Intransitive verb. "Switajewski pointed out the $10,000 cap on state and local tax deductions is temporary, (scheduled to sunset after Dec. 31, 2025), but that Congress could decide to extend it." - Connecticut Business and Industry Association

It's important to note that in this context, set can't be used alone, but must be combined with sun.