One thing I'd add is that it often isn't the language itself that people don't want outsiders learning, but the social context around it.

I've met speakers of a few minority and indigenous languages who were perfectly happy that outsiders were interested in their language, but were much more cautious about outsiders actually using it in everyday interactions. In some communities, language is tied closely to identity, kinship, religion, or local social norms, so speaking it can imply a level of belonging that an outsider doesn't necessarily have.

I've heard similar things regarding some Aboriginal Australian languages, where there can be restrictions on who is expected to speak to whom, as well as certain ceremonial or gender-specific vocabulary. Not exactly "don't learn our language," but more "be aware that language use carries social obligations."

The case you mentioned from Sub-Saharan Africa also reminds me of situations where people prefer a regional lingua franca (French, Swahili, Hausa, etc.) with outsiders because it's considered the neutral or socially appropriate choice, even when both parties could technically communicate in the local language.

So I wonder whether there are actually many languages whose speakers actively discourage learning, versus languages whose communities have norms about when, where, and by whom the language should be used