Thrilled to know I'm not alone here though I've never been more pissed off with an Apple product. Have had a Series 5 since December and started having this issue about a month ago. Had been using a Series 1 for three years prior and never had any battery life problems whatsoever. Before this (and with my Series 1), I could record my walk to and from the office, use Spotify, not-so-covertly text under the table during meetings, log a 1-2hr workout in the evening, track my sleep overnight, and still have 10-20% left by morning.
It first started conking out at precisely 64%, and this past week it's been in the 70s but I haven't been able to keep track of the specific %. When I put it back on the charger, it's a 50/50 split between starting up from zero and from where it left off. The battery gets to 99% within 45 minutes at the most and takes another 20-ish to reach 100%. Once I put it back on, it lingers at 99% for an hour and spends another 4-6 in the 90s, then it's quickly downhill until shutting off in the 60s-70s.
I get it back up to 100% in the morning and really don't use it much during the day now that I'm WFH: check the weather, read texts every so often, record a <30 min walk while running errands a few times a week, do some laps in my 500sf apartment to hit my stand goal. I work out for an hour at 5pm; by then it's in the 80s so I have to give it a quick charge unless I want it to die halfway through the workout (my personal nightmare). Then I give it another charge back up to 99-100% before bed. It's usually in the low 90s when I wake up, but last night was the first time it died overnight so while I'm not sure of the exact battery percentage at time of death, I do know that it shut off after 5.5 hours.
I removed unnecessary apps, turned off Always On and followed all the other standard suggestions for extending battery life, even got a new USB-C charger (directly from Apple) thinking that maybe my USB charger was getting old or something - still no dice. Apple Support performed diagnostics and said it "passed the tests," so I unpaired and erased per their recommendations but the issue persists. I'd really prefer not to have to send it in for repair/replacement since I use it primarily for fitness purposes and am completely insane about tracking activity...