If those are communication apps supported by the bank, that's the idea. Banks have been hit with huge fines for employees communicating over unapproved channels.
One of the problems with the unapproved channels is that the bank can't enforce a retention period. So written messages that are supposed to be kept on record for 10 years or whatever can get deleted. In the event of a lawsuit the bank can be fined for not having the messages.
I understand what they think will happen. But that doesn't necessarily mean they'll be using the banks approved channels etc. You can set up your own private channel in teams and in slack. If people want to communicate without having that info tracked by the bank or company or agency they work for they literally will use another service. Doesn't even have to be teams or an approved one. This article talks about texts and SMS. But WhatsApp and signal and even discord exist (banning one but not the others is playing whack a mole). This seems like the companies trying to keep up with the times without a real plan that considers alternatives.
It's just on work phones, they can do what they like on their personal phones of course. However, if they are discussing anything business related they'll be fired and could be barred from working in the industry altogether.
HSBC etc completely lock down Teams etc, there is no private anything, it's all monitored as long as its on their devices and accounts.
Traders and other front office staff will have every single phonecall, email, IM, voice chat, and video call monitored and stored for something like 7yrs.