The other day I asked for an analysis or at least an UML diagram since we are having quite some troubles and my boss looked disgusted at me for asking such a question. I’m not a professional backend developer, so I don’t know how it works professionally
As others said, in-depth design is often skipped, especially if the dev team started very small. Sounds like your intuition is right, though - the lack of design bit them on the ass when they realized they missed a part.
I have also been laughed at when I suggested a UML diagram in the past. However, it is helpful. For more visually oriented people even more so.
I’d suggest to go ahead and do it, unless your boss is adamant that it is a waste of time. When they see the result they might be happy.