[RPG] Where have the designers of 5e said that they favored rulings over rules in this edition

The designers put their reasoning right in the DM Guide. Experience Points are now optional to help make the game work with more styles of play.

[Doing] away with experience points entirely...can be particularly helpful if your campaign doesn't include much combat, or includes so much combat that tracking XP becomes tiresome.

(DM Guide, page 261)

To support playing without experience points, you can't have a mechanic that requires them, or an economy that uses them.

The rule is an extension of a precedent set by WoTC themselves

The lead rules designer of 5e, Jeremy Crawford, has¹ the power to make official rulings, and frequently does so on Twitter, and in the Sage Advice column on the official D&D site. It's common for him to answer questions with some variation of "if a feature was meant to work that way, it would say so." He has even explicitly stated:

Beware of claims that a rule does something mentioned nowhere in that rule or elsewhere in the core books. There aren't secret rules. (source)

Using this principle, he has made rulings such as:

The Dual Wielder feat doesn't include the benefit of the Two-Weapon Fighting feature. It would say so if it did.
(source)

or

If the grease spell created a flammable substance, the spell would say so. It doesn't say so. (source)

From this, we can derive that barring some explicit clarification from Sage Advice, JC himself, an official errata, or a more specific rule mentioned somewhere else in the game's official material, features in the game are intended to only do what they say. Though, of course, the DM is permitted to make their own rulings and allow spells and effects to do things not directly stated in their description as they see fit.

¹ _{While this status was subsequently backed away from (official rulings only now apply to the Sage Advice Compendium as posted by WoTC, not the tweets from J Crawford), it was true that his ruling was authoritative when this question was asked, and answered; that change in state does not change the overall point.}