"I presume this is the <(1)> shortest decisive game <(2)> between players who (subsequently or previously) played a match for the world championship <(3)> and/or both held the title?"

Condition <1> is easy to understand, now that forfeits and defaults have been excluded.

In Condition <2>, "players who ... played a match for the world championship" is ambigous; it doesn't necessarily mean they played each other. However, <Domdaniel> has since stated that that such was his intent.

Also, the clause "subsequently or previously" implies that the game to be found was <not> played in the WC match between the two players.

Condition <3> is also confusing. "Or" seems to be unnecessary, as no player has yet held the title without playing a match. (Karpov won the title by default, but played title matches later.)

Of course, "World Championship" needs to be defined. If it starts with Steinitz in 1886, then we are restricted to games among these players:

Steinitz--Lasker; Lasker--Capablanca; Capablanca--Alekhine; Alekhine--Euwe; Botvinnik-Smyslov; Botvinnik-Tal; Botvinnik-Petrosian; Petrosian-Spassky; Spassky-Fischer; Karpov-Kasparov; Kasparov-Kramnik

(I don't know whether to count all the recent "champions" who won their titles in tournament play.)

If the world championship goes back to 1851, Andersson--Morphy and Anderssoh--Steinitz could be added. Beyond that--in both directions--soneone else can decide.

Finally, the original statement made no restriction against "rapidplay" events. Should this be restricted to "serious" tournament/match play?

So my example is legal, as would be a (serious) game between Steinitz and Kasparov, but not a game between Steinitz and Short, because Short, although he played a match for the world championship, never held the title.

Anand played and lost a match in 1995, but some people claim he later somehow won the title (without defeating the incumbent champion in a match). Would that make a miniature between Anand and Steinitz valid?

Alekhine vs Euwe, 1921
Euwe vs Fischer, 1957
Tal vs Petrosian, 1962

If you want to include Morphy and Anderssen, then the Morphy vs Anderssen, 1858 game which you pointed out earlier would be a clear winner at 17 moves.