Can't answer your question as to whether the Samsung S10 Plus is on back-order or not but...
So the order number you saw was online only yet nothing has arrived in your email's inbox confirming you placed the order?
The last three phones I purchased through Telus (past 26 months) played out differently each and every time. If memory serves, got a confirmation email for each within a day yet their statuses varied in that I could not rely on webpage to keep me informed. Took 8ish business days for one to arrive on my doorstep with its online status never changing for several weeks after receiving device (still showed order as "pending")... All the devices I had ordered were flagged "Order now and get it within 5 to 7 business days" so not sure if that was the differentiator for me.
I wouldn't be too worried, yet. See what happens this week of Aug 26th then go from there. I'm betting you'll see it by Friday and if not, time to call and give them an earful LOL.
In today's world you'd think technology is at a point where keeping customers up-to-date and well informed should be much improved. Personally not finding that to be the case in many instances as there still seems to be too much inconsistency, stands to reason I suppose as human intervention is still required at some point along the way.