I would also add that these dangling TXT records also build up into a amplification DDoS risk for recursive name servers. When running public recursive servers I have had to strip the TXT records for several domains including but not limited to 500px.com box.com ebay.com google.com hm.com lenovo.com nordstrom.com realtor.com tmz.com wired.com. Take a look:

    for i in $(echo "500px.com box.com ebay.com google.com hm.com lenovo.com nordstrom.com realtor.com tmz.com wired.com");do dig +short +nocookie -t TXT "${i}";echo;done

To strip them in Unbound I used:

    local-zone: ebay.com typetransparent
    local-data: 'ebay.com. TXT "[ddos redacted]"'

My public name servers served a specific purpose so stripping such records were perfectly splendid.