I agree that the dA and AO3 member locking isn't exactly the same, though for me the commonality that it's open registration rather than a select group won out for crafting an internally consistent comm rule. I mean, since it's for pictures and AO3 doesn't allow you to embed images that are protected I'm not sure how well that lock would really work to keep anything protected.

I just wanted to point to a DW example that a privacy consideration from different from what you describe never caused any comment or concern in over ten years. The only requests I had was whether it was okay to respect privacy and creator control less than the norms in my comm, like reccers asking whether it was OK to rec things when the original artist had deleted their Tumblr and pulled their art but you can still find copies others have uploaded (not ok on the comm) or wayback copies (not ok in new recs, but if it was recced before it was deleted the recs stay up as nobody checks gir linkrot, and with the original link people can try whether it got archived).