When I've been in a situation where someone attempts to used unauthorized communication methods, I just ignore 100% of that communication. I've never had to do that to a boss and as others point out, that's sort of risky for your job security. But it would be unethical for you to blatantly violate your NDA and you would be legally liable for damages if something bad happens due to you using unauthorized channels.

I did work at a startup company that was obsessed with using WhatsApp and Skype and ignored my suggestion to use Slack until a CTO was hired who said the same thing and then they switched within maybe 2-3 months.

When I've been in a situation where someone attempts to use unauthorized communication methods, I just ignore 100% of that communication. I've never had to do that to a boss and as others point out, that's sort of risky for your job security. But it would be unethical for you to blatantly violate your NDA and you would be legally liable for damages if something bad happens due to you using unauthorized channels.

I did work at a startup company that was obsessed with using WhatsApp and Skype and ignored my suggestion to use Slack until a CTO was hired who said the same thing and then they switched within maybe 2-3 months.