I think the Scout threshold has some merit – the notion being when you are not on a "greenway" trail or something like that.
However, some can get carried away.
For a 10 mile or less day hike I do have a different list than overnight – some based on the popularity of the trail and likelihood of ending up someplace where I may not be found for a bit. Even that can be a challenge as my son and I hiked a trail last weekend that I though would be a bit crowded and saw only one other hiker over the 8 miles in and back.
So… assuming hike means backcountry in the woods:
– Pocketknife – yes but I EDC a pocketknife so hiking would be no exception
– First-aid kit – yes I always have an FAK with me/us (I almost always hike with my son so sometimes we take his smaller kit – mine is really built around scout trips where I carry some extras for the troop)
– Extra clothing – depends on the weather and time of year and expected length of trip. Summer day hike probably not. Winter day hike yes – some insulation. Overnight yes.
– Rain gear – generally yes on day or overnight hikes.
– Water bottle – yes
– Flashlight – I EDC a flashlight so I always have one including on hikes although I might take something different on a hike – overnight for example I'll have a headlamp.
– Trail food – yes for any backwoods hike – how much depends on the trip
– Matches and fire starters – I have a firestarter and whistle on a lanyard that go on all hikes.
– Sun protection – depends on the hike and the time of year – hiking on the east coast there are a lot of trails that are of the "green tunnel" variety that even in the summer I may not always bring sunscreen.
– Map and compass – almost all the time yes – unless it is a well marked and traveled trail like the pinnacle at Crowders Mountain.