The High Aldwin
Yep, like a lot of 5E... Unfortunately IMO.That's the only real answer we have. Nothing really new for D&D, but 5e really doubled down on it, which makes discussions about the game rather circular.
A guy once posted how shields really shouldn't be part of the don/doff system since they are "carried" (like other items), not "worn" like armor...
Which makes more sense with the use of a hand-held shield as opposed to a strapped-on shield--since 5E doesn't make that distinction, it is entirely up to the DM.