****was a item everyone could build back in the day, as was banshee's veil, and BT was OP.
GA is now the most commonly used along with mercurial schimitar, might be a bit more popular due to the armor on the item, Banshee is AP exclusive in terms of good stats and BT isn't strong enough by itself.
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And positioning is still a thing, i have never gotten away without it while playing anyway.
Only when going like 10,1,12 is it possible to sometimes be lazy, but then then it´s pretty usual to get CC'ed to death if careless.

It used to be supports that were your savior, but now it's just the items that do half the work.
I don't mind that ADCs have defensive options, but with all these shields and damage reduction items going around assassins have to deal twice their hp bar to actually kill them.
Knights vow is a lategame tank item? Doesn't really affect anyone much at the present, but by then the lategame assassins are strong enough to wreck havoc pretty quickly if any opening appears. Eve for instance deals like 1500+ damage with her ultimate alone if it hits.
Relic shield was probably unintended by riot but just like with steraks no one should be surprised x)
BT isnt enough by itself, unless very fed, same with DD except on a few champs like graves or Draven.-.....not that graves is a marksman anymore.
GA lost defensive stats compared to before in exchange for AD, so easier to kill.
locket is a mainly mid/lategame support item as well-
Stasis is available for everyone and is one time use.
Ninja tabi is for everyone and used by everyone when dealing with either a lethality user or by melee against marksmen when CC isn't too abundant.
Now do keep in mind that all marksmen at the present except MF and Ezreal are reserving 3 item slots for crit, one for armor pen and one for boots.
So that leaves one slot open for a true defensive item,boots are more supplementary on most except when dealing with marksmen or lethality.
Only time a marksman doesn't get armor pen is when you don't have a good enough tank or frontline, and at that point you´ll most likely have more damage anyway.
......
That's about it.