You are absolutely right about Wojciech Szczęsny at Barcelona. He was only meant to be temporary fill in when Ter Stegen was hurt but his amazing performance with 14 clean sheets in 30 games really impressed club and earned him permanent contract. While it is unclear if he will be main goalkeeper or backup next season with Joan Garcia also joining most important thing is that Barcelona now has much stronger defense with multiple good goalkeepers which is great for their overall team strength.
What is meant by a permanent contract because from the beginning he was brought in with a contract in the sense that there was no loan or anything else even though the current contract is still on a short-term basis but it is still very good.
Currently Szczesny is still making a good contribution and it would not be wrong for Barcelona to add a new short-term service period or contract for this player because he can indeed be a very good option to be used as an alternative in busy schedule conditions.
Even if you look at the current contract extension and with his previous good performance I think the competition for the line up is now more open where a senior goalkeeper with a lot of experience like Szczesny will compete with young goalkeepers like Pena and that is a good thing for Barcelona.
They realize that Ter Stegen will still be resting so it's a good move to extend the contract this time.