Mikel Arteta can be proud. Regardless of how long they survive in the Champions League this year (they have Real Madrid or Atlético de Madrid in the quarter-finals), they have already written their names in UEFA Champions League history. Yesterday's crazy 7-1 victory over PSV Eindhoven was the first time a club scores seven goals in an away match in the UEFA Champions League knockout stages of the competition. Dutch fans in Eindhoven had to endure their team, who was crowned only once in 1988, being demolished by the English club, and all but confirm their elimination from the competition.
7-1 (or better said, 1-7), with a 6 goal margin, is the largest ever victory by a visitor team in the knockout stages (round of 16, quarter-finals, semis and final) of the Champions League. However, it came short of claiming the largest away win in the Champions League ever in the new era (since 1992): there were three 7-0 (or 0-7), the most recent a Maribor 0-7 Liverpool in 2017/18. All of those were in the group stages.
What is the record the other way around, the largest victory ever recorded at home (and overall) in Champions League? Well, just two weeks ago, PSG already bested Arsenal with a clean sheet: 7-0 victory over their compatriots Brest in the first ever knock-out play-off. And there are better results.
The biggest win ever (by goal margin) was in 1973-74, between Dinamo București 11-0 Crusaders (from Northern Ireland) in the first round. If we are talking about the history of the European Cup before it was known as Champions League in 1992, the largest result in the knockout stages would be a Real Madrid 8-0 Sevilla... in the 1958 quarter-finals. In more recent times, there have been five 7-0 (home victories), all of them in round of 16, except for the most recent one, the aforementioned 7-0 between PSG and Brest in the play-offs, the first time ever that there were played.
Arsenal 7-1 victory was remarkable for being away from home in the knockout stages, but it's neither the largest away victory nor the largest knockout victory in the history of the competition. However, there have been two other occasions of similar results (6-goal margin) in the knockout stages in the Champions League era, both on quarter-finals: a 7-1 Manchester United victory over Roma (at home) in 2006... and the infamous 8-2 victory from Bayern Munich over Barcelona in 2020. That year, due to Covid-19, quarter-finals and semi-finals were a single match in a neutral stadium, so no home or away factors there.