Sorry, but what on earth are you talking about? This whole questions seems based on an imaginary premise. There's no international treaty dictating countries can't sentence people to life in prison, so there's no history of such a treaty.

Almost every country in Europe has one of two forms of 'life sentence' which don't differ much in practice. Either a formal life sentence with a possibility of pardon or parole after a certain number of years, or a sentence with indeterminate length where the convict can apply for parole or a fixed length sentence after a period of time. That'd be for instance 15 years in Austria, Belgium, Germany, Switzerland, 16 in Greece. 12 in Denmark, 18-22 years in France, etc. Norway is not significantly different from Europe here, with eligibility for review for review after a period of at least 10 years but no more than 21, after which the convict can be paroled, released or given a fixed-term sentence, or the indeterminate sentence can be extended for five years at a time.

The European Convention on Human Rights applies, and Norway is a party to that agreement. What the ECHR has determined (e.g. in this case) is that life imprisonment sentences are legal, but but that there must be some opportunity for review within 25 years and periodically after that:

However, the comparative and international law materials show clear support for the institution of a dedicated mechanism guaranteeing a review no later than twenty-five years after the imposition of a life sentence, with further periodic reviews thereafter.

So Norway's laws are entirely in line with that (and if they weren't the ECHR would overrule it). The extent to which people serve out such sentences for the rest of their lives does vary by country. But regardless of whether they do so in practice or not, almost every Western country and European country admits the legal possibility of keeping someone in jail for the rest of their lives.