It is "hopelessly impractical" in the sense that there will always be cases where it's extremely difficult, if not impossible, to implement.
The only impractical cases I know of I excluded already [beta #22312
"Hopelessly impractical" is your claim to defend.
To make a promise in the TOU, that promise must be possible to uphold in ALL cases.
That's like saying that a car warranty must be upheld under ALL road conditions. Ludicrous.
I see no problem with ensuring that the TOS specifically applies only to the messages (and perhaps files and photos) that the person him/herself posted under a given account.
("user leaves, requests deletion of all posts 10 years later"; etc.).
Each account has a fixed posterid value that applies across all groups' messages. That, or the account's email address, can certainly be used to find the user's messages regardless of when the request is made.
To quote Mark, in this thread:
"my lawyers interpreted it as I did not have to delete someone's messages in a group when they deleted their account. It's impractical to be able to remove all traces of someone who has participated in a discussion group."
I never claimed it was a legal requirement.
Nor would I want it to apply just because someone left a group or deleted an account. Deletion at will is already implemented, I only suggest that the TOS recognize and establish that as a desirable policy.
The OP requests that the existing one-by-one deletion be extended to a bulk feature that a group moderator can use. To me this does not seem like an impractical request.
As to "all traces" I agree that's impractical (e.g. my cited exclusions). But it isn't what I or the OP has asked for and shouldn't be what the TOS promises.