Since variable fonts are going to be absolutely HUGE* in 2020, I am pleased to announce that Type 3.2 font editor (Windows, Mac, free and paid versions) now supports the creation and editing of variable fonts.
The first part of a two part tutorial can be found here: https://www.cr8software.net/article010.html
(Note: OpenType feature variations are not yet supported in the current version)
*I know opinions vary
Any implementation limitations we might want to know about? Maximum number of axes? Maximum number of masters?
No limit on number of masters, other than number of named instances?
640K ought to be enough for anyone...
Allan, congrats! I updated my copy and now I'm off to venture into the world of making a variable font.
This includes axis remapping (ie avar table) and extracting stand-alone, static, flat, single-instances and intermediate master outlines (with ranges).