laws that decriminalise acts and sentences that also retroactively free or reduce the sentances of convicts who commited various offences prior to the decriminalisation / sentance reduction
prohibits the retroactive application of criminal law, both in relation to criminal conviction (Article 15(1), first sentence) and greater severity of any sentence (second sentence). It also requires a more lenient penalty to be applied if one is introduced after the offence was committed (third sentence). Article 15(2) preserves the legitimacy of the trial and punishment of crimes ‘according to the general principles of law recognized by the community of nations’, prompted by concerns that the post-Second World War prosecution of war crimes may otherwise be called into question.
But, as Wikipedia in the link above explains, this only "requires the imposition of the lesser penalty where criminal sentences have changed between the offence and conviction."
Many countries have adopted it, but some have adopted it only with reservations. The United States, for example, has specifically reserved and not assented to those provisions of Article 15:
The United States has made reservations that none of the articles should restrict the right of free speech and association; that the US government may impose capital punishment on any person other than a pregnant woman, including persons below the age of 18; that "cruel, inhuman and degrading treatment or punishment" refers to those treatments or punishments prohibited by one or more of the fifth, eighth, and fourteenth amendments to the US Constitution; that the third clause of Paragraph 1, Article 15 will not apply; and that, notwithstanding paragraphs 2(b) and 3 of Article 10 and paragraph 4 of Article 14, the US government may treat juveniles as adults, and accept volunteers to the military prior to the age of 18. The United States also submitted five "understandings", and four "declarations".
Canada has An Act to provide no-cost, expedited record suspensions for simple possession of cannabis (described here).
It also has the Expungement of Historically Unjust Convictions Act (described here).
Statutes can retroactively reduce sentences and allow for those in custody to seek resentencing. See e.g. Terry v. United States (2021) (relating to the Fair Sentencing Act of 2010 and the First Step Act of 2018).
The USA was embroiled in a civil war with separatists. The very act of fighting against the USA was illegal, people were convicted and ineligible for office due to being part of the confederate army. But in 1872, all people who fought and had been made ineglible were forgiven.
Statutes shall determine the rules concerning: ...
- the determination of serious crimes and other major offences and the penalties they carry; criminal procedure; amnesty; ...
The provision has been used at least 10 times (partial list on the French version of Wikipedia) since the Constitution was enacted in 1958.