[2023] How to disable the new settlement changes on a server?

Features.eco:
"SettlementSystemEnabled": false,

You cannot turn it off on an existing world that had it turned on, you need to create a new one, existing worlds will crash.

Absolutely, you can configure everything about the settlements system in Settlements.eco

It is easiest to edit it via the server ui when you start the dedicated server executable, as that will show descriptions for the settings, but the requirements are set here:

"MinCitizensToFoundSettlement": [
3,
12,
30
],

Number of citizens in order Town, Country, Federation.

You can change these settings via the server ui to become effective immediately or via configs, then you need to restart the server. You do not need a new world / savegame, other than when you try to disable the settlements system completely - that does not work on worlds that had them turned on at least once.