Seems like they are moving servers to a new datacentre (it's now being hosted in AWS) which is a much more involved operation than standard maintenance.
Good news is that from earlier today, we can see it was at least running briefly, so I'd guess the bulk of the migration is done, at this point it's probably more a matter of fixing any issues and getting it fully up to expected performance (again, just guessing, I don't know the specifics).
Could still be a bit longer though - even though it's not a change visible to players, this kind of operation has a lot going on behind the scenes.