Hubert’s answer nicely explains the pre-reformed orthographic situation. Anything pre-1996 is likely to follow one of the two former standards (or no standard at all) and would therefore prefer to use an apostrophe here to replace the missing e.
Following the reform, a large number of formerly required apostrophes were either disallowed completely or made optional. Compare the official rules (translated by me):
§ 96: An apostrophe is written in three types of cases:
(1) Proper nouns whose nominative end in an s-sound (-s, -ss, -ß, -z, -tz, -x, -ce) are given an apostrophe in the genitive case if they do not have an article, a possessive pronoun or similar with them.
E1: But: Die Schriften des Aristoteles
E2: The apostrophe is also used if the -s, -z, -x of the nominative is silent: Cannes’ Filmfestspiele.
(2) Words with omissions that would be hard to read or misunderstandable without the apostrophe:
- Das Wasser rauscht’, das Wasser schwoll
(3) Words with inside parts omitted as in:
- *D’dorf (= Düsseldorf), Ku’damm (= Kurfürstendamm).
§97: One may write an apostrophe if words of spoken language with omissions are not clear in written language.
- Der Käpt’n mit’m Fahrrad; Bitte nehmen S’ Platz!
E: To be distinguished from the apostriophe as omission marker is the occasional use of this punctuation mark to clarify the basic form of a proper noun before the genitive’s ending -s or the adjectiv suffix -sch
- Carlo’s Taverne, Einstein’sche Relativitätstheorie.
Omitting the final e in Ruhe does not qualify for any of these cases. The Duden rules go into slightly more detail, by also saying which cases can be considered ‘easy to read’:
An unstressed -e- (translator’s note: shwa) in the inside of a word is dropped:
- Ich wechsle
- trockner Boden
Final -e of certain verb forms are dropped:
- Das hör ich gern.
- Ich lass das nicht zu.
- Leg den Mantel ab.
It is a shortened form of the noun or adjective (translator’s note: or adverb) that is in common use:
- öd (next to *öde)
- Bursch (next to Bursche)
- trüb (next to trübe)
- heut (next to heute)
Phrases which make use of an uninflected adjective or pronoun:
- um gut Wetter bitten
- ruhig Blut bewahren
- Wir wollen sein ein einzig Volk von Brüdern …
I boldfaced the case 3 which is relevant here: Everybody would understand that Ruh is missing an e at the end, so the apostrophe is not required by today’s orthography.