DL does not allow free access to any accompanying travelers, not even spouse/kids. I'm not sure how long it's been that way.
In ~1yr, Centurion will also do away with it for Plat Card holders.

What about UA, AA, AS, Priority Pass? Is there any reason to think any of them might follow suit?

Free lounge access for the family is important for me.
That's what's great about UC membership.
But I became a free agent once UA did away with miles expiration and the pandemic started. Switched to Platinum Card.
But with Centurion's policy change, I'll once again be looking for a new program.