Seems to be testnet3:
https://mempool.space/testnet/tx/e5e38a1708ceca83d2c758780e6037e49932df3700c1b343cc7cae5082cbae06Surprisingly, some people didn't upgrade, and some of them are not going to do that soon:
https://groups.google.com/g/bitcoindev/c/jYBlh24OB-Y/m/XMkcmt7LBwAJHi Sjors,
Thanks for asking. Block/Cashapp relies on testnet3 and can't migrate to testnet4 mostly because of lack of lightning LND peers.
I don't think there are any LND Lightning peers on testnet4. We (Block/Cashapp lightning) use LDK for Lightning, but most of our peers run LND. LND support for testnet4 was just merged, so it’ll likely take time for the LND community to update node infrastructure.
We want to catch peering, business logic and lightning implementations interoperability issues in testnet. Even though LDK supports testnet4, switching now isn’t viable for us until our mainnet LND peers migrate.
Thanks,
Sanket
Which means, that developers can release new testnets as fast as they want, but many users won't jump from one network to another that easily. Also because there is no easy transition, and consensus rules are not wired in a way to encourage it (for example: by burning v3 coins, you won't get v4 coins automatically; that kind of things could encourage more people to switch, but are harder to implement, if coins are not worthless, and cannot be distributed 1:1).
And I guess in faucets, it is a similar situation, as in LN nodes: getting new test coins takes time. Old testnet3 existed for many years, and it will take a lot of time, to reproduce all of that on testnet4.
