True for many people, and indeed Talktalk's FTTP service is data-only; they don't seem to be concerned about losing customers, yet. But a significant minority do still care.
I should have pointed out, users are unlikely ever to receive a letter from Openreach. The user has a direct relationship only with their ISP.
It's possible that Openreach might do generic mailshots or advertising campaigns warning people of the upcoming changes. But only their ISP can tell them "your service is being switched off on date X unless you do Y"
This really needs to be done at contract renewal time. From Dec 2023, any ISP offering a two-year contract will *have* to provide digital voice - or else swallow the cost themselves of switching to digital voice mid-contract. (Or sell voiceless services only)