I would like to learn about any designer explanations about how they came up with the name of the famous city.
One possible hypothesis I have is that the designers were inspired by the planet "Nivil" from the Spelljammer accessory Practical Planetology (TSR 9328), published in 1991, predating the Planescape (PS) setting by 3 years. Nivil is a ring world, and based on its description it is quite clear where its inspiration is coming from. Is it possible that the PS designers wanted to introduce a ring-city and modified the word Nivil into Sigil?
This hypothesis is also partially supported (or at least not contradicted) by the fact that the word is not supposed to be pronounced in the ordinary manner we expect (thanks to @keithcurtis for the reminder). While 'sigil', with its plain English meaning, could be considered to reflect the status of the city as a "magical symbol" or "seal" that keeps the multiverse together, we know that it is pronounced with the hard g, similar to the word cigar. Unfortunately, a 2001 Q&A session with David "Zeb" Cook fails to explain the reason for the choice of the word, only its choice of pronunciation (as a joke). However later in a 2016 interview, he himself can be heard using the regular pronunciation (around 01:08:50).
In short, while I have done some research before asking the question, my speculation is just speculation, and I would like to hear concrete sources. If I were to self-answer with the above, it would be devoid of actual references.
Are there any sources to confirm or reject this hypothesis? Where does the name Sigil come from?