I'm not asking for sympathy, I am simply reporting my experiences. I have played many high level 5E campaigns and the number and rarity of magic items we have is not out of line with those campaigns, including several official WOTC products. We have more than some other 20th level games I've played, probably more than most, but we have less than some others and roughly consistent with the guidelines in the DMG, which are 100 magic items at this level.
A +3 Shield is not even Legendary, it is very rare and that is not one of the problem magic items any way ... the vicious weapons are a far bigger issue. How much 17th-20th level D&D have you played where you don't see a Very Rare item? I think that is a rare game indeed and certainly one which is deviating from the DMG guidelines.
Also you say it is not a problem with the rules; but the reason the party has all those rings is because the rules were specifically changed to allow it. A year ago in 5E someone could only use 3 Rings of Resistance at a time maximum and that is if they were not attuned to anything else at all.
It is changes in the Vicous weapons too, a Rare weapon that will blow away a Legendary +3 weapon.
It is changes in the rules for these weapons that is a primary cause of this.