I don't think it's either the assumption or even a suggestion.
It clearly is an assumption which is enshrined in the Rules. Look at the (median)
Adventuring Day XP table, which is
'the expectation of how much XP the PCs are expected to earn in a single Adventuring day':
Basic Rules for Dungeons and Dragons (D&D) Fifth Edition (5e) - D&D Beyond (dndbeyond.com)
5 x 6th level PC's (for example) have a
median daily XP budget for their adventuring days of 20,000 XP.
Those same PCs have an encounter budget of 1,500 for Easy, 3,000 XP for Medium, 4,500 XP for Hard, and 7,000 XP for Deadly. Meaning the game
expects them to deal with roughly 13 Easy encounters, 7 Medium encounters, 5 Hard encounters, or 3 Deadly encounters (or some mix thereof)
over the course of a single Adventuring day.
As a median, some days will feature less XP, and some days more.
The math of the game (re class balance) is also based around roughly 2 short rests per long rest, and that is also demonstrably true using maths and comparing class balance, and also put forward in the same link as the expected number of short rests per long rest.
As a DM it's up to you to decide how to police that expectation. You can choose to ignore it (but don't complain when classes dont balance out, and your encounters get steamrolled) or you can choose to use Doom Clocks, Rest variants found in the DMG (Gritty realism converting an adventuring day into potentially months of in game time, bracketed by an entire week of downtime) and so forth.
That's your call as DM. But the expectation is there hidden in the maths of the game, and clearly expressed in the DMG around Adventuring days and Encounter building.