Is there a way to create points or lines to depict areas where the speed limit has temporarily changed and have it reflect in the drive times? For example if a portion of a road is under construction and has a temporary reduced speed limit, is there a way for a user to add a point or line to depict the affected area and then the network dataset would pick up and use the reduced speed limit?
The concept of barriers in a network analysis layers is designed for this scenario. You can have point, line or polygon barriers. The barriers can be a restriction or a scaled cost. So you can have a polygon of an area where a weather event is slowing down traffic by 20%. You can load that into your route analysis layer with the a scale cost of 1.2
More info: Barriers—ArcGIS Pro | Documentation
Well assuming that the change from 50 to 25 will make it twice as slow, AND entire line features are affected, you can set the scale cost factor to be 2.0 You can select the affected line features and then load them as barriers. If the slow down happens to start and/or end in the middle of a road then you will have to create an appropriate line feature. The slow down should still be 2.0
If this is a permanent change then you should consider updating the values in the network dataset and rebuilding it.