Bit longer than "decades". Supposedly the Greeks and Romans used charcoal for oral hygiene among other uses and it wouldn't be surprising if it was used earlier than that since people have been burning wood since they discovered fire. You can also make a passable laundry "soap" from wood ash apparently.
While these things are interesting from an historical and experimental archeology point of view or perhaps if you have in mind to live in the wilderness long term without modern convenience, supply chain or a "grid" of any kind, they are definitely marginal in modern backpacking context.
The thing to be careful with when using "off-piste" tooth cleaning unctions is that you can permanently damage the protective enamel of your teeth if you use things that are overly abrasive. It does not regenerate. Charcoal is apparently not recommended for long term use probably for that reason.