Event MPM and FPM would have appeared in the Moodle Docs only once it began to be considered truly stable (e.g. being included in major Linux distributions as default), and hence it was probably not suitable before reached Moodle 2.7.
This is nothing to do with Moodle supporting it though and doesn't mean it wouldn't be suitable for lower versions of Moodle :D
Be cautious though as this is a fairly radical change to your setup and will need changes to your apache config and an informed review of the FPM defaults . You said it's a high load site so make sure you test it all rather than just dropping it in place.
Asides from this I feel I need to mention that Moodle 2.6 is out of support and if you plan on doing something as radical as a move to Event MPM and FPM then an upgrade to a maintained version of Moodle is perhaps worth considering also.