Should the Celtics inquire about Simmons?
I guess Boston is one of the least likely places the Sixers would want to send Simmons. However with his trade value plummeted it's clear that Tatum and Brown are off the table. That does make negotiations easier. The Celtics have all their picks to offer and a fairly okay bunch of reasonable assets (Pritchard, Smart, Richardson, Langford, Nesmith, G.Williams, R.Williams).
If Toronto is interested I'd try to make it a 3-way-deal with Siakam (perhaps not the best fit with either Embiid or Simmons) going to Boston.
Edit: TP, Jvalin (Unfortunately, we cannot trade Smart/JRich/Timelord/Schroder cause of their recent extensions/new contract), hadn't thought about that. This basically makes it a no go fo the Celtics unless somehow some team has a decent interest in Horford.