I am aware of at least 2 telecoms, one publicly traded, that have very little to no IPv6 in their core networks and only use IPv6 when they have to.
Personally I think the design of IPv6 offers very little benefit; supposedly the Dept of Defense/Dept of War holds some 175 million IPv4 addresses, with other companies also holding large allocations - that should have been addressed 25-30 years ago as an administrative matter.
To what end though? 4 billion addresses is not enough on its own, even if they were reallocated from hoarders. I think that NAT and especially CGNAT have been very detrimental to the shape of the internet, where it's nearly impossible to self-host a public service without a VPN of some kind. Needing to pay some company for the ability to host a server that isn't behind NAT is a barrier that doesn't need to exist when IPv6 has a nearly limitless number of addresses.
There are 16 /8's in the class E address space that were never allocated, and 19 /8's (by my count) allocated to individual companies. If you waved a wand and returned all of that space to IANA for allocation, you would have staved off IPv4 address exhaustion by... about 3 years.