"... There have actually been only 8 pitchers that have gotten a win while pitching fewer than 5 innings, all since 1978. Was there a rule change around that time?"

None that I know of.

The rule that allows a starter to earn a win with less than 5 IP is rule 10.17(b) which states:
"If the pitcher whose team assumes a lead while such pitcher is in the game, or during the inning on offense in which such pitcher is removed from the game, and does not relinquish such lead, is a starting pitcher who has not completed
(1) five innings of a game that lasts six or more innings on defense, or
(2) four innings of a game that lasts five innings on defense,
then the official scorer shall credit as the winning pitcher the relief pitcher, if there is only one relief pitcher, or the relief pitcher who, in the official scorer’s judgment was the most effective, if there is more than one relief pitcher."

This means that in order for a pitcher to win such a game his team can not have pitcher more than 5.2 innings and he must have pitched at least 4 innings.

Using the team pitching game finder we can find that from 1954-1977 a team won a game while using 2 or more pitchers in less than 6 innings on 9 occasions. In all of those games the starting pitcher pitched either less than 4 innings or at least 5. It wasn't a rule change, just a quirk of history.

I checked to see if any of the ones that Eorns listed occurred early in the 1995 season. I remember that there was some talk that year of requiring a starter to go only three innings to get a win as major league baseball eased back after a late start to the season and an abbreviated spring training at the end of the strike. But I don't recall if this was ever done (although teams had expanded rosters for a few weeks), and none of the ones listed here are from that time period.

I don't know if there's a minimum for spring training games, but I do often notice that a starter who has gone less than five full innings is the winner.

I'm surprised that teams only go 119-135 with a bare minimum quality start. Either my perception is wrong, or 2009 had some random fluctuations.

I love that teams have a 100% winning percentage if their starting pitcher works 2 and 2/3 innings and gives up 11 runs!

Player
Date ▴
Tm
Opp
Rslt
App,Dec
IP
R

Wilbur Wood
1978-05-12
CHW
MIL
W 4-3
GS-5 ,W
4.1
3

Bob Forsch
1985-08-24
STL
ATL
W 7-0
GS-4 ,W
4.0
0

Mike Griffin
1987-07-20
BAL
CHW
W 4-1
GS-5 ,W
4.0
1

Richie Lewis
1992-07-31 (2)
BAL
BOS
W 4-3
GS-5 ,W
4.1
3

Larry Luebbers
1999-10-03
STL
CHC
W 9-5
GS-4 ,W
4.0
5

C.C. Sabathia
2001-06-01
CLE
NYY
W 7-4
GS-4 ,W
4.0
4

Chris Michalak
2006-09-22
CIN
CHC
W 4-2
GS-5 ,W
4.1
2

Andrew Carpenter
2009-05-16 (2)
PHI
WSN
W 7-5
GS-5 ,W
4.1
5

Provided by Baseball-Reference.com: View Play Index Tool UsedGenerated 1/4/2010.