Just to clarify this, Unraid NEVER has "phoned home" for paid licenses. All key validity checking is done locally. Trial keys, yes, they verify expiration time, but paid keys don't expire, and aren't checked externally.
If you use your own domain for remote access and don't sign in to the UPC, nothing will change with 6.10 except for the sign in screen for UPC will be present and nag you to install the my servers plugin. The my servers plugin is optional, so if you don't want the unraid.net domain or flash drive backup on unraid's servers, don't install it.
As far as the key purchases, trial keys will require sign in. Paid keys can still be handled externally if desired.