Calculating isochrone for linestring

You could do this using QGIS and the TravelTime plugin. You can turn lines into points using method 2 in this tutorial https://wiki.tuflow.com/index.php?title=QGIS_Convert_Lines_to_Points. Once you have the line you can create isochrone shapes from every point, do this using the Time Map feature, there's a walk through of how to do it here https://docs.traveltime.com/qgis/tutorials/basic-tutorial once completed you can merge these isochrones into a single layer. Here's an example of how we created this.

You could also do this using just the API but would have to turn a line into points first https://docs.traveltime.com/api/reference/isochrones

Disclaimer: I work at TravelTime

You can also do it with the QNEAT3 Plugin: https://plugins.qgis.org/plugins/QNEAT3/

I have a line which is 7.7 km long, represented by 1112 points, so ca. one point every 7 meters. To get the points from a line, use densify by count: https://docs.qgis.org/3.10/de/docs/user_manual/processing_algs/qgis/vectorgeometry.html#densify-by-count

Than I have a network (red lines).

Using QNEAT3 Plugin, I selected Iso-Area as Polygons (from Layer) with the points and the network as input and selecting the fastest path (time optimization) with a default speed of 5 km/h (fast pedestrian) and a size of Isa-Area of 3600 (calculated in seconds, thus 1 hour) and a contour interval of 600 (thus 5 minutes). This is the output I got - calculation took around 12 minutes: Polygons with iso-area of 5/10/15 etc. minutes (represented with white lines) and an interpolated raster, styled here with the magma color ramp (yellow to black).