@WTF_Huitzo Well Respawn has gone on record to say that it won't come back because the "core" of the game is "teamwork", and that they had saw solos mode to be "an unhealthy aspect for the game".
Another point against it, was that "some legends were team based". Legends like Pathfinder, Wraith, and Bloodhound were highly picked and most often used as they "can be played as a solo". Legends like Lifeline were not, as "their kit evolved around teamwork".
They added no-fill to pubs as an alternative to a solos mode, so players can be by themselves, but they would be the only ones missing out on the "teamwork". (Teamwork is in quotes because let's be honest, when was the last time our randoms worked as a team, and they weren't a duo pre-made?)
I personally would love to see a solos mode again. However, it needs a system that detects teamers. Back during the Iron Crown event, you would see 2-5, (maybe even more), of the same legend fighting together. This was my major issue with the mode.