Hi Starlight,
Unfortunately, SoftEther VPN will not be returning to DietPi for the moment.
Implementing and supporting Softether VPN is too time consuming and I have limited time I can devote to DietPi as it is.
The other reason is due to its end-user setup with the client software. Its over complicated and we can’t automate that side for the user.
You can however install SoftEther VPN server manually on any DietPi system. This guide may be helpful.
Here is the DietPi source code for installing SoftEther VPN server, before it was removed.
Install code: DietPi/dietpi/dietpi-software at 2b3c38f0182a12085347153bb979b3030e0fd70d · Fourdee/DietPi · GitHub
Config code: DietPi/dietpi/dietpi-software at 2b3c38f0182a12085347153bb979b3030e0fd70d · Fourdee/DietPi · GitHub