The canada post workers told me that stamps last for forever.

I could be wrong but I doubt they stopped honouring them. A) they were sold as permanent, B) although not widespread use of stamps are considered general currency.

old school note: Used to get them as change sometimes at the postoffice or small shops, and could use them toward purchases

I've never heard that and I'm pretty sure I used two in the last year, with no issues. I can't find an article about it. Can you link one?

The permanent ones are permanent, the ones with an amount written on them require that you add the amount missing by adding extra stamps.