From the news reaching the rounds, PSG is sending an appeal to UEFA regarding their Tuesday Champions League Semi final first leg fixture against Arsenal. Before now UEFA always allows 5% of the tickets allocation to the away fans in it's competition, Arsenal has a stadium capacity of 60k of which 5% should be 3k but they only gave 2,500 tickets to PSG fans which is not up to the stipulated number. When they reached out to Arsenal, Arsenal said they can't give more than that citing it for security reasons because the London marathon will be held on Sunday. That's a silly excuse, are they trying to shut the away fans or what. Why do they want to bend the rules now? Out of 60,704 stadium capacity you are still finding it difficult to allow just 3,000 away fans in.
https://www.football365.com/news/psg-appeal-uefa-unfair-arsenal-champions-league-semi-final-turmoilIt is clearly unfair to PSG because the 5% is already set by UEFA and always required that 5% of the stadium's capacity go to away fans, so there is no reason to justify what arsenal did to PSG in this case. and regarding the security issue, I don't think that is something that needs to be used as an excuse for this, because if they really care about security then they can tighten the security more instead of reducing the 500 seats quota from PSG.
It's easy to handle such issues. Since Arsenal has given limits to PSG fans in England, if Arsenal is to play the second leg in France, let PSG also limit their fans number. Limiting the number of fans wouldn't add any impact to the outcome of the game because the strongest will definitely win the match. However, since Arsenal is playing at home, they have more advantage.