I've been using openapi/swagger for generating my clients and honestly think it creates a mess of files and manually keeping clients up to date is not a huge task. I'm not sure that I buy into "your client libraries should be generated automatically" as a blanket statement.

Granted, APIs aren't updated often, but when they are, generated clients can save a lot of time. I can update a generated client in half an hour and be very confident of its correctness, whereas a manually constructed client would take hours to update and test.

Of course, the generated client is not very convenient, being a 1-to-1 mapping with the API. You build a high-level client with the more common operations on top of the generated one.

It's possible to do this in a way that gets you the best of both approaches, with some up-front planning.