Like I posted above, the “visual spacial storage” that PC stashes have is probably the issue.
Why the hell did they even decide to go with “jenga”/“inventory Tetris” storage system for PC anyway? Just the dragging and dropping item visual would take up system resources to animate needlessly when you already know where the item is going based on what windows are open and what button is pressed.
I think the “issues” were not actually “technical” with the addition, but I can’t follow the dead link from the blog post you linked to find out even the most basic info. It still sounds like more of a “we don’t know how to add another ‘Tetris level’ to this mini-game storage we have” than an actual “we can’t add more without breaking the performance”.
Maybe they should apply the Switch optimizations to other consoles and even PC then to give the game a new lease on life and more performance headroom. They must already have the code documented. It could just be a “reverse port”. Then PC could even get optional gamepad play, which is what is preventing me from playing on PC most of all.
Oh well, it’s a lost cause as the game is in maintenance mode, even with new sets and updates to keep a trickle of new sales coming. I only really remembered this game existed, after not even having a console to play it, after years because of the Nintendo Switch version; hell, I had an Xbox One temporarily just before the Switch that I never even looked for Diablo 3 for because I didn’t remember it at the time.
Edit:
After some investigation I found a few posts, notably by MVP MissCheetah, that players were NOT HAVING ANY ISSUES with an actual implemented and functioning additional stash tab(s) on the test realm.
It seems the tabs were canned for the same nebulous “too much work” reason I assumed, and/or they possibly found their server load increasing too much when they had no hope for an upgrade on a “if it ain’t broke, don’t upgrade it” server. That server excuse seems a bit unlikely considering the players testing it had no issues and that would hardly be an issue if they also improved the way stashes worked at the same time anyway.