15 years ago I think Arduino was the best choice for educational purposes. I still think it's a great choice now. The fact the IDE and board are basically the same as they were 15 years ago, means you can figure out how to set everything up once and focus on teaching, rather than PC trouble-shooting. Which, for basic concepts, or younger kids, is great. And if they find a 5 or 10 year old video on how to do something, it's still relevant.

If I were putting teaching materials today - I would pick something like Micro python. The down side is it isn't as "canned" a solution, meaning there might be something new to figure out every so often. Which means you spend more time helping people trouble shoot why something isn't working, instead of teaching something useful. On the up side, Python is pretty much the introductory language of choice, today. With lots of available materials.

That's not to say Arduino was perfect. Far from it. Just easier to do, and more consistent over time, than other options.