Flying while encumbered

[Bellberith]

Can't find anything in the rules about mounts and encumbrance.... can you fly or have a mount fly while you/it are encumbered?

[Daishain]

I don't think there is a specific rule, which would put it in the realm of DM judgement.

Based on how I would handle it, it depends. (and this is in turn supported by the official rules of previous editions)

Physical flight, as in actually beating wings to generate lift, would be impossible while encumbered, although a glide can still be achieved.

Being encumbered while in magical flight reduces your speed, but otherwise does not hinder you.

As for mounts, you being encumbered while riding a mount has zero effect on the mount. BUT, if the total load of the mount is more than its limit, including the stuff you carry, then it does become encumbered and the above applies.