Seeing as VST is a proprietary format, is it legal to create and share free VSTs without the permission of or paying royalties to Steinberg?
You’ll find some info here:
I have no direct link to the licensing agreements. Maybe this stuff is enclosed in the download files.
The licensing agreement is included with the SDK, it says so right there.
Nobody has to pay anything to Steinberg to develop and release VST plugins. Mailing them should also not be necessary unless you’re planning to sell a plugin.