I'd categorize this as a "Fix" since it prevents the undesired behavior of artificially increased rate of emigration

please add named constants instead of magic numbers. example:

note to self: we really really need to fix up and merge DFHack/dfhack#3907

this will leave last_cycle_tick as nil in current games with saved state. It would be better to normalize the value here so you can remove the check for nil in event_loop.

I suggest a pattern like this:

    state = get_default_state()
    utils.assign(state, dfhack.persistent.getSiteData(GLOBAL_KEY, state))

now that we have more than just enabled here, I suggest making this clearer by putting the state in a table.

local function get_default_state()
    return {enabled=false, last_cycle_tick=0}
end

state = state or get_default_state()