Leaving aside the subject of drift in free-running VFOs (pre PLL & TCOs ) one regular feature of old CW transmitters, particularly where they were simply high-power oscillators, was the chirp you got as the frequency dipped on key-down whilst the HT re-stabilized.
In an effort to simulate this an audio keyer is being prototyped to bring back those old days. The video shows the first hack ( 20 minutes today ) . The sound is terrible as I have not added low pass filtering to the square wave output and it is playing on a piezo sounder. It is also dual-tone (selectable by design).
The next step is to change the chirp algorithm to natural log as this better mimics the capacitive recovery effect on old HT keyed circuits.
Update – some drift on this one, a different chirp and amplifier stage was added (3rd-order RC LPF too ).