Most offline systems, and many systems used only for using trusted code (which obviously bars anything with a web browser, and most internet-connected personal computing devices, among other things). Applications that equate roughly to "Scientific number-crunching" for example are generally systems where security doesn't matter much.
There's a case to be made for gaming, and the mitigations if I remember correctly definitely ding performance on quite a few games, but because most computers used for games have significant amounts of sensitive information on them, among other things, I don't think it really holds well.
Benchmarks (I really dislike linking to this site, but given it did a bunch of benchmarks, it's not the worst thing, I guess):
'Up to 50% performance loss!' is obviously clickbait, and I disagree with a lot of the wording & conclusions & so on, but there's definitely some workloads where the trade-off makes sense.
Warning about those benchmarks, though: they're of a rather old kernel version, I imagine they tank performance less now. However, given that 5x has been performing way worse to my knowledge than 4x, 4x is probably what most people would be using for these sorts of things.
Are systems used for "scientific number-crunching" really applicable in this case? While some departments may have those on a separate network, not connected to the internet at large, I have never heard of a system employed for research tasks being completely air gapped. Otherwise, accessing and working on data would be prohibitively harder. Would such a trade-off be worth the potential gains by deactivating mitigations?
Also, may I ask why you dislike Phoronix? I personally enjoy their articles and the benchmark suite they have developed seems very well-rounded and transparent. I wouldn't count the statement concerning the ~50% increase in time it takes to complete a certain task on 4.20 as clickbait, considering it was never used in the linked articles title to hock readers and gain clicks.
Honestly, I have yet to see a large and popular enough use-case that allows for a both completely air gapped system, whilst also heavily benefiting from disabling mitigation to such an extent, that an admin couldn't just lock up the flags required. If I, as an admin, made the conscious choice of going so far as to disable these patches, I would also want to at least re-read whether this is truly significantly advantageous, rather than copying a line from a website with no context or further information on the current state and impact on performance.
It'd be nice to be able to easily boot into, or toggle into, a performance optimized, disabled mitigations environment to do something while offline.. many computer uses don't require being connected to other computers. I've gotten into the habit of hotplugging my Ethernet connection, personally.
You can actually do that fairly easily, just add the parameters linked to a second boot entry in GRUB.
However, I would very much not advise doing so, as I still am unaware of any task that can both, be done without the need for a network connection, while also being significantly slowed down by the mitigations, after recent improvements to the kernel and software. Basically, the potential benefit is very low in a lot of tasks, whilst requiring additional security measures (ideally fully air-gapped) and that you reboot the system every time you'd do such a task.
Also note that, in theory, just being temporarily offline may not shield from being exploited fully.
As an example (the only case that I've identified personally), if your curious, I have a (windows; Intel q6600) box that I use for gaming occasionally. Single player game I like, Total War: Shogun 2, runs at about 55 fps (benchmark) pre-Meltdown/Spectre/etc. Now it gets ~22 fps. I can use https://www.grc.com/inspectre.htm to toggle some mitigations to get it playable again.
