Given the tone of the message, I expect "non rapid" to mean with a much longer time frame, and, most importantly, the release must be controllable by the user.
This is pining for a world which no longer exists, because security patches must be applied to anything that matters immediately.
Of course you do need a strategy for maintaining multiple versions of something in that case instead of just forcing the latest version of everything you do on users whether they want it or not. With the lock-in effects of the always-connected web applications of today we all know which option makes more money.
I take some comfort in knowing that eventually most of the developers that force unwanted changes on their users will probably see their market share erode and lose their dominant positions. It's just unfortunate that we have so many quasi-monopolies in both essential and niche software products that it's going to take a long time for that to happen.
This world does in fact still exist, this is how most OSS works!
CD is a deploy strategy saying the aspiration is to do one deploy per commit, rather than batching commits and deploying once per N weeks or whatever. It has approximately zero to do with UX and product design/iteration approaches.
If you're not familiar with the general arguments for why CD might be desirable, a good place to start would be the books Continuous Delivery [https://smile.amazon.com/Continuous-Delivery-Deployment-Auto...] or Accelerate [https://smile.amazon.com/Accelerate-Software-Performing-Tech...]. The TLDR would be that most deploy problems in practice turn out to be because the process is manual, infrequently-used, and/or unreliable, and by practicing "if it hurts, do it more often" you are forced to iron out those kinks. For example you can't deploy automatically without a good acceptance/smoke test suite, and without solid DB migration testing/linting/practices, to name just two of many. You could just build all these things without trying to do CD, but in practice it seems that it's quite hard for organizations to prioritize this work in isolation, and the goal of CD seems to provide a fairly clear map to a much higher-quality deploy process. The practice of deploying more frequently to flush out issues, is also quite unintuitive as you note.
Here's a (probably out-of-date) article on how FB did deploys in 2017: https://www.infoq.com/news/2017/09/facebook-release-scale/ -- batched commits and staged canary rollouts automatically cut and released every few hours.
If you have external contracts that are onerous to validate then it can be quite expensive to build high-fidelity internal representations of those contracts. Could still be ROI positive to do so!
In most cases the contracts are either internal or simple (non-temporal) external contracts like a REST API, so most orgs benefit from moving in this direction.
We came from very different lands I guess. Batching 50 commits with no easy way of knowing which commit caused the issue seems scarier to me, in case of an issue. And if you code correctly and have warmup requests, your deployment could be without any downtime. Obviously there will be some extra resource usage because of unready resources.
Doing this in production is being disrespectful of your users. Users are not waiting for your PRs to be approved and merged, they can be as delighted by your new button’s shade of lightgrey if it is released some days (even weeks !) after.
The connection is that if the commit changes how the product works in a way noticeable to the user, then the user must suffer the changes frequently. Since changes incur a cost to the user, it's requiring the user to constantly be paying those costs.
If the deployment happened less frequently, then the user would incur the cost less frequently, and perhaps in a way they could schedule when to take the change on so it is the least disruptive to them.
Most software nobody cares if it works or even is possible for untrained humans to use, and it shows.