Indeed. I seem to recall Gerry Fiennes's book recording how they put in the connection between the ex-Midland and GC route as part of the rationalisation of the Sheffield area in, probably, the 1950s or early 60s. For those that don't know, there were four double junctions and the connections of opposite hands intersected via four diamond crossings. Trains via Doncaster not serving Rotherham Central would take the right-handed connection and those via Rotherham Central towards the Dearne Valley would use the left-handed one. The Mexborough-Swinton reinstatement made the right-handed connection at Aldwarke redundant.
Yes, the original Swinton curve (on a different alignment which appears to have been a tighter curve) was closed in 1965 when the new connections at Aldwarke Junction were put in, part of the Sheffield area alterations for the new Tinsley marshalling yard. Without any Swinton station to call at, trains from Sheffield to Doncaster could use Aldwarke Jn to access Mexborough via Thrybergh. It seems odd to think that the GC and LMR were previously parallel to each other but not connected at Aldwarke Jn.
There's a short chapter "Aldwarke Junction - From Inception to Closure" in the book
Tinsley and the Modernisation of Sheffield's Railways explaining the changes (the book appears to be out of print but is worth buying if they ever reprint it - £34.95 is pricey but it's a large book with 240 pages, well-researched and filled with interesting information and photos). The British Transport Films "Freight and a City," which is currently on YouTube, also covers freight developments in Sheffield in the 60s and is well worth watching for anyone who hasn't seen it.
The original layout of the junctions at Mexborough is interesting. I've attached a schematic map of the 1950s Mexborough area signal boxes which appears in a little book called "Yesterday's Railwayman" by D.A. Newbould. This book is a reminiscence by the author who worked in the area, mainly as a signalman but also in other roles, and gives a good flavour of the railway in South Yorkshire, covering the period from 1956 to 1970. Worth buying if you can pick up an inexpensive second hand copy (it was published in 1985). Visible from the current Swinton curve, there is still a bridge over the A6022 at Mexborough on the trackbed of the redundant south to west curve from the GC line towards Wath.