There are enough examples in the games such as Rebellion and Empire at War, which show orbital bombardment is useless against planetary shields. This is also the case in Empire Strikes Back, where the shield generator needed to be taken down before the Empire's attack could begin. In the case of ESB, we have to assume this isn't a full planetary shield but only covered the Rebel base, as evidenced by the Empire landing ground troops outside the perimeter and then moving them under the shield. The Decipher Customizable Card game explains this in detail: it wasn't a full planetary shield (it only extended to the "Ice Plains" 5th marker) but was still powerful enough to be immune to bombardment.
Taken from the Advanced Rulebook:
The Hoth: Main Power Generators site produces an
energy shield which is “strong enough to withstand any
bombardment.” This shield protects much of Hoth for
the Light Side by preventing many forms of Dark Side
deployment and movement; however, the Dark Side
may deploy or land beyond the shield and “march in” to
Echo Base. When the Main Power Generators site is
face up on table, the energy shield covers all Echo
Base sites and the first three marker sites. If the 5th
marker is on table, the shield will reach the 4th marker,
and if the 6th is on table it will reach the 5th marker. The
shield will never extend beyond the 5th marker. At
shielded sites, the Dark Side may not:
deploy vehicles, starships or characters (not even
spies) unless specifically allowed to deploy to shielded
take off, land, shuttle, docking bay transit, perform a
Bombing Run, or use abilities that relocate cards (e.g.,
Elis Helrot, Chief Bast); or
add power to battles as a result of starships controlling
the system (e.g., from the Hoth system location, a
Victory-Class Star Destroyer or Fear Will Keep Them
The energy shield does not protect the Light Side from
a ground assault “underneath the shield.” At shielded
sites, the Dark Side may
move vehicles and characters from site to site;
deploy and use creatures, weapons, Effects, and other
cards normally; and
perform any actions not otherwise prohibited by these
The energy shield does not restrict the Light Side in any
way (because conceptually the Rebels can activate and
deactivate the shield to allow their own forces to pass).
When the energy shield is not active, both sides can
deploy and move to Hoth normally.
See Locations - Battlegrounds, Ch. 9.
So, whilst there's no canon/in-universe reference I'm aware of explicitly stating that the Endor base was shielded, I think it's a fair assumption that they had a shield around the facility; it's almost implied by the need for the rebels to land out in the forest, beyond the edge of such a shield.
As to why the base commanders weren't immediately put on alert when the rebel shuttle landed out in the forest beyond the shield instead of at the landing platform patrolled by the AT-AT is probably a topic for another question.