You are mistaken with respect to ownership of lighting in Salsibury - it is all mainatined on a county basis - the city has no ownership of any assets since the unitary Wiltshire Council was created more than a decade ago. Lighting was not a devolved operation when Salisbury City Council existed.
There isn't a a"huge amount of installs on residential roads that don’t use nodes". All legacy lighting was converted from conventional photocell to Telensa RF switching around 10 years ago when part night/selective switching was brought in.
You are also mistaken with respect to nodes and SELC cells. All the LED Axias use a new version of Telensa RF CMS switching cell. See my extensive descripition here - post of 3rd Nov 2020.
viewtopic.php?f=5&t=144&start=130Some SOX lighting was left, predominately on concrete columns that are slowly being sleeved, or installations which were in difficult locations where a "quick" swap wasn't possible or where traffic management would be required. These will be changed as they burn to exinction.
The market square installations are not new, again they have been there since at least mid 2000s so would be considered life expired especailly given energy costs. Cosmo is seen as an obsolete lamp given its high replacement cost.
Salisbury was only partly converted to LED in the summer of 2020. This was based on areas where the lamps were oldest with about half the city completed. The remaining parts were done in the summer of 2021. At the same time Highways England - or whatever they are called now, replaced the lighting on the A36, which took well over 18 months to complete.