The OFL FAQ has a entry on this:
Question: 3.6 Do I have to make any derivative fonts (including extended source files, build scripts, documentation, etc.) publicly available?
Answer: No, but please consider sharing your improvements with others. You may find that you receive in return more than what you gave.
So no it’s not a legal requirement, so it’s really your decision.
Rather than making it a strict requirements in the license because the definition of what constitutes a modifiable font source can vary quite a lot and it could have become a blocker, we tend to view publishing extended sources (more than just the generated fonts) as a recommended best practice: SIL does that with the fonts it develops for example. And you can find plenty of OFL-licensed fonts with extensive sources (and the full buildpath) on public git repositories from other foundries and designers. Various people find that it makes sense if you’re going to develop an open font to provide the full sources with the related build scripts and documentation. But again it’s up to you.
I hope that’s clearer.