Is Pinging an ISP’s provided DNS servers blocked nowadays?

Is Pinging an ISP's provided DNS servers blocked nowadays?

I notice that BT(a UK telephone company and ISP), provides the DNS IP of 81.139.56.100 and 81.139.57.100

If I ping www.google.com I get an IP v6 address back.

And if I try to ping those IPv4 DNS IPs 81.139.56.100 and 81.139.57.100 which my router lists as having gotten, then it doesn't work.

But interestingly,

the command "nslookup bbc.co.uk 81.139.56.100" works

also ping -4 www.google.com returns an IPv4 address.

And of course I could change to use Google's 8.8.8.8 or 8.8.4.4 which can be pinged and also work.

But, is this a new thing now of ISP's blocking people from pinging their DNS servers?

I notice that ipconfig for my connection says DHCP enabled, and lists an IPv6 address that tracert shows resolves to my local router. So my local router is my DNS server, and that access those DNS IPs.

Is this a new thing?

When did this change occur?