Well one way to decrease damage
is by not decreasing it but scaling up tankiness where needed, that could be done large scale by changing the way how the formula for armor/mr works
But also by increasing/decreasing base stats where needed
The other option is tampering with penetration and other stats like crit making it more situational like tying in bonus armorpen with crit and moving it away from the last whisper items then reducing crit damage foring more desison making between crit, flatpen or AD or added utility or defense
That's just an example of splitting up universally overpowered damage into be strong vs one target but weaker vs another with your damage or gaining power elsewhere vs having pure damage
The same could be done with magic damage make voidstaff gain more utility stats over AP for example and force a choice between having power vs all targets to having power vs defensive builds gain massive power vs tanks but gain barely anything vs squishier targets
There are lot's of ways to go about dealing with damage being too high but imho the best way would be to move away from damage builds being generally effective and force decision making.
right now i mostly see this in tanks, you can be effective vs dealing with basic attack damage through getting reductions from randuins and tabi for example but then you have to use 2 slots these items do a lot less vs ability damage or raw high AD attacks and are completely useless vs magic damage and of very little use vs true damage they also are vulnerable to penetration to a degree.
You gain effectiveness vs 1 type of offense but you sacrifice slots making you not so effective vs all other ways people can deal with you you want maximum protections then you have to sacrifice things like cdr or certain actives or other utility
I think we need to go the same route for damage as well