For short-term rental put into service in 2024, can I use the De Minimis safe harbor for <$2500 purchases before it was put into service or only after?

Only purchases made after the property was listed as available to be rented are eligible to be expensed under the Safe Harbor Election.

Any costs you incurred before you listed the property increase the basis of the property and should be entered as separate Rental Assets, subject to depreciation. 

IRS Pub 527 Adjusted Basis: "You must increase the basis of any property by the cost of all items properly added to a capital account. [This] include[s]…[t]he cost of any additions or improvements made before placing your property into service as a rental that have a useful life of more than 1 year."

This rule includes start-up costs as well. Under section 195 of the Internal Revenue Code, certain expenses incurred as start-up expenditures in an active trade or business are deductible up front up to $5,000, with the balance amortizable over fifteen years. However, Section 195 does not apply to rental property because renting is not considered an active trade or business, but rather a passive activity.