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From what I've seen: I highly doubt it.

Edit to add: This might spur on a few more to start doing that, but people are quick to forget/prioritise other areas. If this keeps happening then it will change.

“Their own” can and often does mean something hosted on a major cloud provider (whether they manage it in-house or pay a vendor for their system)

Yeah, perhaps. I don't know how many folks host mirrors. Most places I've worked for didn't, though this is anecdotal.

I would say most people would say it't best practice while a minority actually does it.

Seems related to size and/or maturity if anything. I haven't seen any startups less than five year old doing anything like that, but I also haven't seen any huge enterprise not doing that, YMMV.