Deschler's Precedents. Chapter 14. Impeachment Powers.
... Contemporaneous with the drafting and adopting of our own Constitution was the impeachment trial of Warren Hastings in Great Britain. Hastings resigned the governor-generalship of India before he left India in February 1785, 2 years before articles of impeachment were voted by the House of Commons for his conduct in India. The impeachment of Hastings was certainly a fact known to the drafters of the Constitution.
George Mason, in discussing the impeachment provision on September 8, 1787, in the Constitutional Convention, makes a clear reference to the trial of Hastings ... Prof. Arthur Bestor states that--
American constitutional documents adopted prior to the Federal Convention of 1787 . . . refute the notion that officials no longer in office were supposed by the framers to be beyond the reach of impeachment.
Bestor specifically cites the constitutions of two States - Virginia and Delaware - which were adopted in 1776.
Bestor also cites a statement of John Quincy Adams, made in 1846 after he left the White House, made on the Floor of the House:
I hold myself, so long as I have the breath of life in my body, amenable to impeachment by this House for everything I did during the time I held any public office ...
https://www.govinfo.gov/content/pkg/GPO-HPREC-DESCHLERS-V3/html/GPO-HPREC-DESCHLERS-V3-5-5-2.htm