When it become a felony ? When Trump made removing classified documents a felony....

Maybe you should actually read the statute instead of posting a photo of it from some leftwing website.
18 U.S. Code § 1924 - Unauthorized removal and retention of classified documents or material(a)Whoever, being an officer, employee, contractor, or consultant of the United States, and, by virtue of his office, employment, position, or contract, becomes possessed of documents or materials containing classified information of the United States, knowingly removes such documents or materials without authority and with the intent to retain such documents or materials at an unauthorized location shall be fined under this title or imprisoned for not more than five years, or both.
(b)For purposes of this section, the provision of documents and materials to the Congress shall not constitute an offense under subsection (a).
(c)In this section, the term “classified information of the United States” means information originated, owned, or possessed by the United States Government concerning the national defense or foreign relations of the United States that has been determined pursuant to law or Executive order to require protection against unauthorized disclosure in the interests of national security.
https://www.law.cornell.edu/uscode/text/18/1924
How nice of Congress to exempt themselves from this statute. But the key bits are in sections 'a' and 'c'. Section 'c' specifically pertains to "classified" documents (i.e. documents affecting national security). Such documents have
ZERO to do with this Maralago search. And then there's Section 'a' which applies only to officers, employees, contractors, or consultants of the United States. Trump was none of these when he gained possession of these documents shipped to him by the GSA after he left office.