The DMG and XGTE give pretty good guidance on exactly how many and what kind of magic items to give the party in 5e.
There is no "rule" in 5e saying you should not give out magic items. You absolutely need to give out magic items unless you are constantly making specific considerations in the monsters and challenges you select.
What is true is that the CR on monsters do not assume that the party will have Specific Magic Items at Specific Levels. AC is not tuned assuming all 8th level characters will have +2 weapons, for example.
