USCIS published the final STEM rule discussed in this blog post. In publishing the rule, USCIS gave it an effective date of May 10, 1016. The only exception to this is 8 CFR 214.16 which is effective from May 10, 2016 until May 10, 2019 (when it sunsets). This section states that those OPT applications previously filed for a STEM extension that are still pending on May 10, 2016, will be issued an RFE to show that they qualify for an extension under the new rule (i.e. that there is a training plan, etc.).
This is important for those with a pending OPT STEM extension application as you may need to provide additional documents to show you qualify under the new rule. However, if USCIS does adjudicate the OPT STEM extension before May 10, 2016, then that extension would be granted under the old rules.
Please remember, as always, this blog does not offer legal advice. If you need legal advice, consult with a lawyer instead of a blog. Thank you.