The idea of thieves cant does have some historic and real life roots.
We'd call it street slang now. The idea of street or criminal cultures having a dense web of slang, jargon and innuendo to get their point across in ways that wouldn't be immediately obvious to average people. If you didn't know "gangster" or "street gang" language from movies and TV, would you know what a mafioso was saying when he was saying to "take someone for a ride"? How many people would have no idea what someone was trying to buy when they were looking for "grass" or "acid" if not for popular culture?
Treating it as a completely different language in 1e and 2e, one that was class-exclusive to thieves, that was the problem.
When you look at a dictionary of historic Thieves Cant, you'll recognize a lot of the words, because over time they entered standard English and slang terms. Words like "brat", "kid" and "chip off the old block (as terms for a child) all started in 18th century thieves cant.
Just check out a dictionary of the actual Thieves Cant:
18th Century and Regency Thieves Cant
I'd definitely support the idea of Rangers being able to learn Druidic, if they can learn druidic nature magic after all. Then again, I was never comfortable with the fact that you're a outdoorsman and archer means you just happen to pick up a lot of Druidic magic casting along the way. I think the idea of Druids having a secret language was a holdover from their inspiration from the Celtic priests of antiquity, and how they kept their written language to themselves both within the priestly class of society, and within Celtic culture.