The variation margin requirements of the uncleared margin rules were to start globally on 1st March. However due to a backlog in the required “re-papering”, most jurisdictions signalled that the rules would be delayed using various legal mechanisms (see here). This delay is now ending in many jurisdictions and the rules will take effect for covered market participants (in the EU, many energy companies will remain exempt). This article from ISDA highlights the large amount of work that has been carried out and how much more ready the industry is. This report on The OTC Space by Nick Stafford provides more background.
However, those who are not ready, and who have entered into contracts after March 1st may have some issues, as reported here on The Trade web site.