iserith
Magic Wordsmith
But doesn't it become more than a perceived issue at a certain point? A level 1 or 2 PC is quite a bit more likely to die on a single turn than even a level 4. It's just a simple matter of HP and defenses.
It's even worse if a most of the group is moderately high level and for whatever reason you bring a new PC into the mix. If the group is going off to fight the dragon that level 1 character can either tag along and become a charcoal briquette after the first breath weapon attack or they can stay at the inn while the real adventurers go off and do their thing.
Unless of course low level PCs simply have plot armor.
The biggest level difference I've seen is seven from the lowest-level PC to highest-level PC in that group. In that campaign, the 1st-level PC joined up with the party and delved into the dungeon which they had been clearing for some time. He was a melee guy, but figured he'd hang back a bit at first and do ranged. The group encountered a couple higher-level monsters (can't recall which exactly), but given that level of the dungeon, probably in the range of CR 5 to 9. A fight ensues. The PC makes some ranged attacks, which hit (because monster AC can be low even at higher CRs) while observing what the monsters can do, then he runs in to do a Help action to help a heavy hitter overcome some disadvantage which leads to the other PC killing the last monster. The XP from that encounter causes him to immediately level up to 2nd and get halfway to 3rd.
I remember this (except for the monsters) because everyone talked at the start of the session how it was suicide to bring a 1st-level PC to the area of the dungeon they had been exploring. Only, what we found was that if the player is savvy, he or she would do just fine. The player was very happy with his accomplishment and made 4th level by the end of the night. That was a very interesting data point that stuck with me, even a couple years later.
Of course, that is only one data point. Most level differences I see are one to three levels in our two weekly campaigns and in my one-off adventures with player pools. Sometimes these are across tiers where one would expect a noticeable impact. Still, the players or characters are able meaningfully contribute. And we're not into plot armor. You will earn your victories and survival in our games. If you know your character is going to be particularly fragile in a certain fight, you act accordingly or you die. That's just player skill. That hill giant you mention? Get outside of its rock-throwing range - even a light crossbow can do that. Spend Inspiration to offset the disadvantage due to range. Or get within 80 feet, but further than 60 feet, attack normally, then drop prone. It either attacks at disadvantage due to range or due to prone. Attack and run behind total cover, if you can. Have another member of the party get adjacent to the giant so its ranged attacks are at disadvantage. And so on and so on...