After checking 2017 ATP Rulebook
(Internet Archive) I found out that there are indeed restrictions for high ranked players which disallow them playing at challenger level without receiving a wild card.
Here is quote of the relevant section:
7.07 Play-Up Regulation (ATP Challenger Tour Tournaments)
A. Restrictions
1) Players positioned 1-10 in the Emirates ATP Rankings twenty-one (21) days prior to the first Monday of the ATP Challenger Tour tournament are prohibited from entering, accepting a wild card and/or competing in an ATP Challenger Tour tournament.
Players who would have been, had they and all other players entered, a direct acceptance on the original acceptance list for a Grand Slam are prohibited from entering, accepting a wild card and/or competing in an ATP Challenger Tour tournament in the first week of the Grand Slam tournament.
2) Players positioned 11-50 in the Emirates ATP Rankings twenty-one (21) days
prior to the first Monday of the ATP Challenger Tour tournament are prohibited
from entering but may receive an ATP-approved wild card in order to compete
in the Challenger tournament. The Challenger Supervisor will make wild card
determinations consistent with the limitations outlined below. Players positioned 11-50 are also prohibited from entering, accepting a wild card or competing in ATP Challenger Tour tournaments that offer less than $75,000/€64,000 in on-site prize money plus Hospitality.
3) ATP Challenger Tour tournaments scheduled the same week as an ATP World
Tour tournament may offer wild cards to players positioned 11-50 in the Emirates
ATP Rankings who have received approval from the ATP on-site Supervisor according to the following breakdown:
$150,000/€127,000 in prize money up to (2) two wild cards
$125,000/€106,000 in prize money up to (2) one wild card
$100,000/€85,000 in prize money up to (1) one wild card
$75,000/€64,000 in prize money plus Hospitality up to (1) one wild card
$75,000*/€64,000* in prize money no wild card
*Plus Hospitality
4) ATP Challenger Tour tournaments not scheduled in the same week as ATP World
Tour tournaments, or during the 2nd week of a Grand Slam, ATP World Tour Masters 1000 Miami and Indian Wells, may offer wild cards to players positioned 11-50 in the Emirates ATP Rankings who have received approval from the ATP on-site Supervisor according to the following breakdown:
$150,000/€127,000 in prize money up to (4) four wild cards
$125,000/€106,000 in prize money up to (4) four wild cards
$100,000/€85,000 in prize money up to (3) three wild cards
$75,000/€64,000 in prize money plus Hospitality up to (2) two wild cards
$75,000*/€64,000*in prize money no wild card
*Plus Hospitality
I will ad also link to this blog post from 2015, which analyses success rate of Top 50 players in challengers: Benoit Paire and Overqualified Challenger Contenders.