However, e.g. whitehouse.gov refers to "President Clinton" in its description (for the act of being born, not for his active role as president).

https://www.whitehouse.gov/1600/presidents/williamjclinton

Where I live, Finland, a "President" is the official title for former presidents (there are now three alive) so I should actually address them as such. The current one is formally referred to as "President of the Republic". I can't really check how this is with Russia.

A friend of mine moved in next door to then President of Singapore. Even got visited on couple of Christmas dinners.

Neither of them new protocol for the ex-presidency thing and after President left office, upon mutual agreement neighbour now calls him "Uncle".

I'm not convinced. Multiple other sources say you retain the title for life and also recommend addressing the person as such. Obviously it's a matter of etiquette and not right or wrong, though.