I don't understand the difference between 1 and 3.

I may have used "free as in free beer" in a wrong way, what I meant with 1 was there are no additional costs to the current or new users (the rates for the services on offer stay the same).

For 3 (as was in the example), the cost of the DDoS protection service is directly added to the rates of services on offer.

OVH was quite blatant in this, as it had offered an optional DDoS protection service for a fixed rate of 3€/mo (this was a few years ago, exact details might be hazy). After they had a large network overhaul (with major interruptions), they simply raised the prices by 3€ and advertised the new, "free" DDoS protection service which was included in all of the services.

3 is very unlikely, hosting plans generally go down over time, not up. And Linode, like most metered hosting services, where you're billed hourly, don't normally distinguish pricing for new versus recurring customers.

Instead the price would just go down more slowly. So the price drop that you'd otherwise expect is paying for the new features.