To be technical, 80 is multiplexed with I-20 through the western half of Clinton. Once it separates, 80 has it all to itself until Meridian. There is a significant amount of traffic that travels on 80 through Jackson and especially through Pearl and Brandon, so my guess is that decommissioning and multiplexing it with I-20 wasn't necessary. If my guess is wrong, then I'll stand corrected.
I've had to so some research on all of this as part of my work duties. Here is what I've found:
I-20 from Clinton (Exit 35) to Bovina (Exit 11) was originally built as the "new" US 80 in the 1950's. The posted overpasses at Bovina (Exit 11) and Edwards (Exit 19) are a part of the road's pre-interstate era. MDOT turned the old US 80 over to Hinds County and Warren County. Later on, those sections of new US 80 were widen/rebuilt to interstate standards to carry I-20. I-20 east of Exit 35 was built on a new alignment as a purpose-built interstate highway. The same is true for US 80/I-20 from Clay St (Exit 4 A-B) to Washington St (Exit 1A). Old US 80 from Exit 4 to west of Exit 11 is still on the legislative designated state highway system as MS 822. MDOT recently added MS 822 trailblazers next to the incorrect US 80 trailblazers on this section.
I-55 from near the Hinds/Copiah County Line to I-20 (including State Street from I-20 to US 80) and from Woodrow Wilson to US 51 @ Ridgeland was originally built as the new US 51 in the 1950's. The old roads (State Street north of Woodrow Wilson to the new US 51, Terry Rd from MS 18/Raymond Rd to Cunningham St in Terry, Cunningham St from what is now the East Frontage Rd to Railroad Ave, Railroad Ave from Cunningham St south to the current East Frontage Rd, and old US 51 from the end of the Frontage Rd section to the county line) were all turned over to the local governments (City of Jackson, Town of Terry, Hinds County). Later on, the new sections of US 51 (except between I-20 and US 80) were rebuilt to meet Interstate standards to carry I-55. Frontage Roads were necessary to access businesses that had opened on the new US 51. On I-55 north, the exit ramps were tied into the two way frontage roads. This was all converted to Texas style one way frontage roads in the late 80's and early 90's. I-55 south has not been rebuilt since its conversion from US 51 with the exception of (1) reconstruction of the Byram exit and (2) a permanent median wall after years and years of a temporary wall.
I-55 between I-20 and Woodrow Wilson was built as a purpose-built Interstate highway. Since it was a brand new road, there were no businesses and no need for Frontage Roads. US 51 remained on State Street until the mid 70's, when the city of Jackson and MDOT agreed that the city should control and maintain State Street from the bridge north of US 80 to Woodrow Wilson. The "spur" leg of US 51 from I-20/55 to US 80 is now basically a glorified on/off ramp.
Once I-55 was completed, US 49 moved from its previous alignment on US 80 and US 51 to I-55 between I-20 and Woodrow Wilson. In 1987, MDOT and the city agreed to move US 49 to I-20 and I-220 and give the old US 49 to the city. Unlike US 51, MDOT pulled all of the US 49 signs off of the old road and the old alignment on the interstates rather quickly after the turnover. MDOT did not include the old US 49 as part of MS 149 because it was not bypassed by a new section of US 49 - I guess