This second level was used both as a dormitory and a dining room by the garrison (which was made up of about twenty soldiers), as suggested by the fireplace, the old utensils and the garrison officer’s room.
Oxford English dictionary
Here the subordinate clause introduced by as is elliptical, and that's why the auxiliary verb of the subordinate clause is omitted.
The un-elliptical version of the subordinate clause is it is suggested by the fireplace, the old utensils and the garrison officer's room
You are right that the sentence could also be written correctly as the following -
This second level was used both as a dormitory and a dining room by the garrison (which was made up of about twenty soldiers), as is suggested by the fireplace, the old utensils and the garrison officer’s room
as (conjunctive or relative adverb)
The clause introduced by as may be reduced by ellipsis of its verb and other elements to one or two important words, leaving as as a quasi-connective:
(a) Between an adverb or adverbial phrase in the principal clause and an adverb or adverbial phrase constituting the subordinate clause.
It suffices me to say . . . that men here, as elsewhere, are indisposed to innovation. Emerson, Lit. Ethics.
(b) Between the principal verb or its subject and the sub-ordinate subject or object, which becomes equivalent to a predicate appositive or factitive object after the principal verb, a, meaning ' after the manner of,' ' the same as,' ' like,' 'in the character or capacity of,' etc.:
The audience rose as one man.
All these things were as nothing to him.
He has been nominated as a candidate.
Hence in constructions where the appositive clause depends directly upon the noun:
His career as a soldier was brilliant.
The construction as a quasi-predicate appositive or factitive object after a principal verb is usual after verbs of seeming or regarding.
This gentleman was known to his contemporaries as a man of fortune, and as the author of two successful plays.
The subordinate clause introduced by as is often not dependent grammatically upon the principal verb, but serves to restrict or determine the scope of the statement as a whole. Such clauses are parenthetical, and usually elliptical, some of them, as as usual and as a rule, having almost the idiomatic rarity of an adverbial phrase.
The streets were narrow, as is usual in Moorish and Arab cities.
The train is late, as usual.
Reference - The Century dictionary