I have completely rewritten this comment, so Reed Crosby's comment may not be exactly coherent with my comment.
I found what I think is an/the early note on reddit about this at https://www.reddit.com/r/iphone/comments.... So, although we don't know what exactly triggers the throttling (battery cycles or capacity or age or ?), we do know a way with either CPU Dasher X or comparative Geekbench scores to see if you are being throttled.
I did a cursory security check on CPU Dasher X (no big red flags when searching the web, it looks like there is open source code on github for some version of it), downloaded it, and found that I am not throttled (CPU speed very close to max). The coconutBattery application (runs on your computer but will check you phone's battery when connected) says my phone's battery is still above 95% / 1545 mAh at only 257 cycles, even though the phone is 15 months (about 450 days) old. I would guess it is capacity or cycles based, as it seems like I might get throttled on age alone. I'll comment back here if I notice a change and remember to come back.
Update at the 27 month mark for my iPhoneSE:
I have been throttled or had sudden shutdowns occasionally for a couple of months. My battery status since then (January 2018)
2018-11: 83-85%, for a couple of readings around 400 cycles. No problems.
2019-05: 84%, 483 cycles. Consistently down here, but still okay.
2020-01: 83%, 626 cycles, about when problems started
2020-03: 83%, 661 cycles, still usually gets me through the day unless I use it a lot.
I just ordered a replacement battery kit. In part because of the problems for the last couple of months, but mostly because I just dropped my phone in the water. I fished it out quickly, dehydrated it for about an hour a couple of times but saw some symptoms when I checked after each of those. Dehydrated it over night and it seems fine, but I want to crack the case, check on it, and clean up. Figured the battery replacement went along with that.