backstage/.changeset/thirty-pets-fry.md

@backstage/frontend-plugin-api, @backstage/core-plugin-api

@backstage/frontend-plugin-api	@backstage/core-plugin-api
minor	minor

BREAKING PRODUCERS: The IconComponent no longer accepts fontSize="default". This has effectively been removed from Material-UI since its last two major versions, and has not worked properly for them in a long time.

This change should not have an effect on neither users of MUI4 nor MUI5/6, since the updated interface should still let you send the respective SvgIcon types into interfaces where relevant (e.g. as app icons).