Great to see that there are still many people interested in this QoL suggestion
You can find the full discussion in the Optimal Walling topic.
If you want to see it in-game, you can test it with the Age of Salamanders scenario (as mentioned by @SlinkyRobin8549 ).
I also made an interactive JavaScript tool only for this L-shape issue (with comparison to the current implementation ; open it in full page for the optimal experience):
There are in fact 4 different configurations when joining two wall end points. In the JavaScript tool, you can use the mouse wheel (or right click) to switch between these 4 configurations, similarly to the placement of gates in DE.
You can play with the aforementioned JavaScript tool to see something similar (but using the mouse wheel for 4 configurations instead of 2).
Nice, this would fit well with the Dijkstra Tile Indicator suggestion