There are 2 repositories under opentype-variable-fonts topic.
Variable font example in OpenType-CFF2 & TrueType formats
Online font tester. OpenType features, kerning, variable fonts, language support.
Repo for discussion and review of proposals for registration of OpenType design-variation axis tags.
Variable Font Collection Test
White On Black VF versus Black On White VF
An OpenType font binary diff tester
Extensions, scripts, plugins, templates for the FontLab font editor
Width & Vertical Width VF
Convert OpenType COLR or COLRv1 fonts to SVG