Icon doesn't list chit for specs. They've been doing the same for years.
Only thing i know about the Icon coil springs is that they have a 19.25" free length. Don't know about the rear coil springs in the new Rebel.
Can you take one of yours out and measure ?
This is the only thing Icon list in their website about their springs
ICON 2009-2023 Dodge/Ram 1500, 1.5” Lift, Rear, Dual Rate Coil Spring Kit ICON 2009-2023 Dodge/Ram 1500, 1.5” Lift, Rear, Dual Rate Coil Spring Kit - Icon Vehicle Dynamics
www.iconvehicledynamics.com
Don't know much about 5th gen suspension, like you it took me a while to find some specs but for the 4th gen and made a little excel spreadsheet to try to help the 4th gen community in the ramforum site. i ran the Icon springs for over one year and have 1st hand experience with them, but again my truck is not a 5th gen or 5th gen Rebel so i can only help you with the general idea.
My 14 crew cab Mopar specs factory coils were 18.25" and in theory should have a 206lbs/inch linear rate.
When i bought the Icon 1.5" coils Icon used to advertised they were 25% softer than stock in their 1st rate and 75% stiffer on their 2nd rate. I see they've changed that verbiage and now say little softer than stock, still 75% on 2nd rate
I got 1 1/4" lift with them stock weight
3/4" with 450lbs in the bed
It takes about 800-1000lbs to make them sag 1", stockers will sag 2.5" with 1000lbs
I've never ever felt those Icon springs being harsh, never, even under load. Have no idea what the factory Rebel coil spring rate is but i doubt you'll be able to say that Icon coil springs ride harsh.
I ran the Icons with Rancho 9000 shocks that were valved for a RAM 2500 (always looking to improve things offfroad, i bought Icon coils cause i wanted longer shocks, so the 3rd gen 1500 or 2008-2010 or 12 2500 have the same rear shock eyeletes as ours and they fit like a glove) then later switched to Kings 2.5 suspension front and rear. Icon coils rode better than stock in my case, less wobbly under load, same as stock or better in the plush department.
I'm curious about the 2020 Rebel coil springs cause it seems from your conversation with Icon that they should provide 1/2" in a 4th gen truck and be a little stiffer than stock.