The "In Session" function is not new, all that is new is the warning message, added [here](https://github.com/geany/geany/commit/295f59da91d2a2c08a5289d56d30f3caa2d2d5...).
The "In Session" replace although it has been available for at least 15 years (as far as I looked).
There is no option to disable it, just don't use it, or say "no" to the warning dialog.