1955–1991: Knockout format, one club per country (the league champion) plus the defending champion.
1955: Some countries were represented by a team not the domestic champion.
1956–59: The domestic runner-up was allowed to compete in cases when the domestic champion was also European champion.[12]
1991–1993: Three knockout qualifying rounds, group phase with two groups, two group winners meet in final, one club per country (the league champion) plus the defending champion.
1993–1994: Knockout semi-finals added following group phase.
1994–1997: One knockout qualifying round, group phase with four groups, group winners and all runners-up to eight-team knockout phase, one club per country (the league champion) plus the defending champion.
1997–1999: Two knockout qualifying rounds, group phase with six groups, group winners and two runners-up to eight-team knockout phase, up to two clubs per country.
1999–2003: Three knockout qualifying rounds, two group phases with eight first-phase group winners and all runners-up moving to four second-phase groups, second-phase group winners and all runners-up to eight-team knockout phase, up to four clubs per country.
Since 2003: Three knockout qualifying rounds, one group phase with 8 groups, group winners and all runners-up to 16 club knockout phase, still up to four clubs per country.
Since 2009: UEFA introduced a play-off round in qualifying round of the Europa League and Champions League.
So yes there were no group stages before. Repeating has become so much harder.