A few pieces of code

--current version
script.on_load(
    function()
        global.subtick = 0
    end
)

function on_sneak(event)
    local you = game.players[event.player_index]
    if (sneak == false or sneak == nil) then
        sneak = true
    else
        sneak = false
    end
    you.print("Key pressed (debug) sneak=" .. tostring(sneak))
end

function on_tick(event)
    if subtick==nil then
        subtick = 0
    end
    subtick = subtick + 1

        for _, player in pairs(game.players) do
            if (subtick >= 2 and not player.walking_state.walking) then
                subtick = 0
                sneak = false
            end

            if player.walking_state.walking then
                if sneak then
                    player.print("sneakin")
                    player.character_running_speed_modifier = -0.5
                else
                    player.print("no sneakin")
                    player.character_running_speed_modifier = 0
                end
            end
        end
end

script.on_event("sneak_modifier", on_sneak)
script.on_event("crawl_modifier", on_crawl)
script.on_event(defines.events.on_tick, on_tick)

-------------old version below
--  toggles only when I press the key, needs to chnage when I hold / relese the key
script.on_event("sneak_modifier",
    function(event)
        -- global.sneak = false
        local you = game.players[event.player_index]
        if (sneak == false or sneak == nil) then
            sneak = true
            sneaki = "true"
        else
            sneak = false
            sneaki = "false"
        end
        you.print("Key pressed (debug) sneak=" .. sneaki)
    end
)

function on_tick(event)
        for _, player in pairs(game.players) do
            if player.walking_state.walking then
                player.print("Walkin")
                if sneak then
                    player.print("sneakin")
                else
                    player.print("no sneakin")
                end
            end
        end
end
script.on_event(defines.events.on_tick, on_tick)