Yes, the national default limit for "restricted" roads (those with lighting at frequent intervals) will change from 30mph to 20mph on September 17th 2023. And there is "guidance" from Welsh Government regarding what factors could justify keeping the speed limit at 30mph. The primary consideration is whether there are significant numbers of pedestrians and cyclists travelling along or across the roads. This is all detailed in the Welsh Government Exceptions Guidance. We have created this flowchart diagram to show you how this works :-

It is our opinion that even if the road is used by pedestrians and cyclists then a well segregated cycle and pedestrian path with controlled crossings may provide a basis for setting a higher speed limit than 20mph. However any "robust and evidenced application of local factors" would need to take into account other pertinent characteristics such as road width, traffic volumes and presence of housing, community buildings, schools, etc. But note that the "guidance" is only "guidance". The need to fully take into account the needs of vulnerable road users was part of the previous guidance on setting local speed limits. 

The needs of vulnerable road users must be fully taken into account in order to further encourage their mobility and improve their safety. Setting appropriate speed limits is a particularly important element in urban safety management, with significant benefits for pedestrians and cyclists. Similarly as vehicle speeds are generally higher on rural roads, collision severity and the risk to vulnerable road users are also greater. In both situations
speed management strategies should seek to protect local community life.

The absolute responsibility and authority for setting a different limit for a street from the national speed limit lies with the local highway authority. They are well placed to consider the needs of all road users including those who are most vulnerable and the need to preserve local community life. Local authorities in Wales are setting exceptions to the national 20mph limit for some main roads and these may be viewed at the Welsh Government website.

Note that it is important to recognise that its is often "main" roads which pose the biggest risk to vulnerable road users due to the volume of interactions. Hence lowering the speed will disproportionately reduce the likelihood and outcome of any collisions.