Savings plans have mostly replaced RI's https://aws.amazon.com/savingsplans/faq/ They basically offer the same bonus's with greater flexibility. More on reddit.com

I've got 3 more months on my t2.medium reservation and would love to switch to t4g for the next 3 years. Any chance this will happen soon u/jeffbarr ? More on reddit.com

We run quarterly assessments to figure out how many instances are running 24x7, and then further break that down to understand the long term strategy for those particular systems / platforms. If we expect it to be on more than 7 months, we buy an RI for it. We have a lot of traditional on premises platforms that have mostly lifted and shifted into the cloud, so RIs make a ton of sense for us. Likewise, even more elastic and scaling platforms still have minimum baseline compute running at all times and use auto scaling to scale up during peak periods. It still makes sense to buy RIs for those systems, as there’s always X running 24x7. Managing them isn’t too hard. Once you buy them, it’s set and forget until the next quarterly review. RIs don’t tie to a specific instance, so even if we decommission a platform earlier than expected, we can shut those instances down and use the RIs on other systems that were more recently spun up and not covered. We use cloudhealth to do this mostly automatic for us. They provide RI recommendations, we look them over and act accordingly. We have roughly $2m a year in spend and RIs helped us cut that down to about $1.2m effectively overnight. We do 1 year no up front exclusively. More on reddit.com

Reservations, like savings plans, are a billing abstraction. In this case, you are committing to pay for a specific instance size, in a specific instance family, in an AZ or Region, in exchange for a discount. You do not have any assurance that you will actually be able to spin up that kind of instance however. Reservations also have some amount of flexibility, but upwards. For example, if you purchase 2 m3.large reservations, they will primarily apply to 2 running m3.large instances. If none are running, it will try to apply against a running m3.xlarge instance. Reservations are more limiting than savings plans and I would personally preference savings plans for their flexibility. More on reddit.com