A considerable amount of real business is now conducted in personal messaging apps. A customer sends a photograph of the thing that needs doing. A price is agreed in a few lines. A supplier confirms a delivery date. Somebody approves a change while standing in a car park. It is fast, it is informal, everybody is already there, and it works. For a lot of trades it has largely replaced email for day-to-day dealings, and arguing that it should not have is not much use, because the reason it happened is that it is genuinely better for the moment of the conversation.
The problem is what happens afterwards. That conversation is the only record of what was agreed, and it lives on one person's personal phone, in an app tied to their personal account, mixed in with their private life. Nobody else in the business can see it. It cannot be searched from anywhere else. If that person leaves, the record leaves with them, and asking a former employee to hand over their personal message history is both awkward and, quite reasonably, refusable. If the phone is lost and the history was not backed up, the record is simply gone. And when a dispute arises six months later about what was agreed, the business may find that its evidence is a thread somebody deleted to free up space.
There is a second, quieter cost. Because the conversation is invisible to everybody else, the business cannot function around it. Somebody else answering the phone does not know that a change was agreed on Tuesday. The person doing the job does not know the customer asked for something different. The invoice gets raised for the original scope. None of this is anybody's fault, and all of it comes from the same property: the agreement is in a place that only one person can see, and the rest of the business is operating on information that has quietly gone out of date.
The workable answer is not to ban the practice, which will fail, but to make one habit routine: when something is actually agreed - a price, a date, a change of scope - it gets confirmed into a place the business owns. An email, a note on the job record, a line in the shared system, whatever the business already uses. Thirty seconds, in the business's own words, sent to the customer so it is confirmed rather than merely recorded. Keep the chat for the conversation; put the conclusion where it belongs. That way the speed is kept and the record stops depending on one person's phone and one person's continued goodwill.