We're certainly not a dead project. We're very much alive and well! :) Although the Joomla appliance itself requires a rebuild (to include a newer version of Joomla - ETA next week hopefully). PHP will continue to be 7.0 for at least the next few months.

It sounds like you may be coming from the Windows world? When "new = good" and "old = bad"?! In the Debian world, things work much differently (TurnKey is based on Debian). Debian (along with other popular Linux releases such as Ubuntu and Red Hat) prioritises stability (and security) above latest release. That's why most of the world's servers run Linux, not Windows!

Whilst PHP7.0 is EOL by upstream (i.e. the PHP developers) it is still alive and well in Debian (what TurnKey is based on)! :) It will continue to be supported until sometime during 2022!

FWIW I answered a somewhat similar question regarding PHP in further detail recently and I think it's likely relevant to you too. Have a read here.

The next major release of TurnKey (v16.x) will almost certainly be based on Debian 10/Buster which will include PHP7.2 by default, but that isn't expected until later this year (mid year?)

If you are adamant that you want PHP7.2 (and are willing to manually monitor it for security updates and apply those updates as required) then I did give some hints on how to do that in another post here. Here's the relevant bit:

If you are sure that you want/need PHP7.2, then I recommend that you use Ondřej Surý's DEB.SURY.ORG. Ondřej is one of the main packagers for PHP within both Debian and Ubuntu and is therefore the next best thing to "official" packages. You can find the instructions for setting up his repo here. Then you should be able to install php7.2 like this:

apt update
apt install php7.2

You can leave the default PHP installed and use the Debian alternatives system to define which ones are used by default. There are some relevant notes in Ondřej's docs here. Although personally, if you have no need for PHP7.0, then it's probably better/easier to just purge the default PHP libraries before you install PHP7.2. I haven't tested, but something like this should do the trick:

apt purge php php7.0

If you choose to go that path and need a bit more of a hand, please don't hesitate to ask.