Almost 12 years after the last update to SMB3 Workshop a new Editor emerged with the goal of rising from its ashes.
With permission of the original Workshop creator, Hukka, a reimplementation project grew into a complete new Editor providing several quality of life improvements for Level creation.
These include:

  • selecting and moving of multiple objects at once
  • cut/copy/pasting of objects
  • undo and redo of most manipulations
  • more intuitive level pointer manipulation
  • support for vertical levels
  • instant header editing

and more…
Although not all features of Workshop are implemented yet and some quirks have not yet been fixed, after 5 months of development a beta version of the 1.0 release is available for a wider audience.
Download:
There is a Windows and Linux standalone version available with Mac users having to go the slightly higher effort route of downloading Python and one dependency as described in the Readme of the source code.
Contact:
For enthusiasts who want to follow the releases more closely the GitHub page offers the source code, the newest versions and an issue tracker for bug reports and feature requests.
For direct contact, reach out to Michael on the SMB3 Romhacking Discord
Contributors are also welcome, as Python is easy to pick up, compared to other Programming Languages.

https://github.com/mchlnix/SMB3-Foundry

http://www.romhacking.net/forum/index.php?topic=28826