Hello,
Microsoft announced the idea of Sets a few days ago, you can take a look here https://www.youtube.com/watch?v=3lEjuU-XFHg.
Of course the idea is the same as in the old KDE 4 and was abandoned rather than improved in plasma 5!
Window Tabbing it's Great feature


I hope developers will return to this feature and improve it and take advantage of the previous code.
happy day

If it's not around through plug-ins then it would be cool to bring it back.

Having functionality that was once available go missing is not a good feature.