I do most of my aging in kegs. It works without problem and meads age fine in them. Some meads (think heavy-tannin berry melomels), like some red wines, may benefit from some oxygen uptake during the maturation process. Usually, racking it a few times as it clears suffices in my experience.
Most wines and most meads do not benefit from oxygen exposure after the fermentation is finished. The normal aging process does not require oxygen if you don't have a lot of tannins to polymerize, and most meads don't.
I normally flush the headspace in my kegs to eliminate oxygen, but I typically use Argon or Nitrogen so I don't get a carbonated result.
If if a keg is pressurized and seal with the air still in it, depending on how much air is in there, you could have excess O2 pickup. If your keg has 5 gallons in it, with minimal headspace, flushing the keg probably makes no difference. If you have 1 gallon of mead, and 4 gallons of headspace, flushing is probably necessary to minimize the chances of spoilage (which may be a bigger risk than direct damage) or oxidation. Some meads, particularly delicate fruit melomels, probably need to have the space flushed more than a traditional mead.
One potential negative about flushing headspace is that you do flush out a lot of aromatic elements that fill that space. How important that is, I'm not sure, so I try to limit flushing to the time I am transferring batches.
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