I would expect you would be best served by starting a new game (required or not). Remember that you can always choose to keep playing your old saves by downgrading (or not upgrading) your version of the game. Every old version back to 7 something is available there if I recall correctly.
The way it's worked in the past has been you have to intentionally "opt-in" for the experimentals.
In Steam program R-click over top of game (7dtd) choose "Properties", in pop up window go to "Betas" tab and in drop down for "Select the beta..." choose ... something obvious. It's not currently showing any betas.
If you don't do that then your game will not be automatically updated to an experimental build.
To stay running A18.4, even when a stable A19 is released, you can go through the same steps, right now, and opt-in to "alpha18.4 - Alpha 18.4 Stable"