Linear growth is not really growth, in a financial sense, as growth is when the derivative (rate of change) is increasing/decreasing. Linear growth by how I interpret it, would have a derivative of zero, i.e. the rate of change is constant, or geometrically speaking, a straight line.

Financial terminology doesn't distinguish between linear and exponential. Linearly growing companies would simply be assigned very low (close to 0 / positive) CAGRs.

I agree. For example, if you spend exactly $1000 on advertising a day, and have no network effects, you will get linear user growth.